Objectively, the most terrifying pre-K/T extinction land animal was the Tyrannosaurus Rex.
I thanks Con for making this wonderful debate, now swiftly to my argument:
Although I disagree that T-Rex is the most terrifying land animal, I will try my best to defend my position, good luck.
T-Rex is terrifying:
"Tyrannosaurus rex was one of the largest meat-eating dinosaurs that ever lived. Everything about this ferocious predator, from its thick, heavy skull to its 4-foot-long (1.2-meter-long) jaw, was designed for maximum bone-crushing action.
Fossil evidence shows that Tyrannosaurus was about 40 feet (12 meters) long and about 15 to 20 feet (4.6 to 6 meters) tall. Its strong thighs and long, powerful tail helped it move quickly, and its massive 5-foot-long (1.5-meter-long) skull could bore into prey." 
Look at these teeth and jaws:
Imagine this dude coming at you at full speed:
And now I conclude my argument.
While the T-rex was undoubtedly a formidable predator, there are many pre K/T extinciton animals that were far more terrifying than it. I will describe three such animals to prove that Tyrannosaurus was not the most terrifying animal.
1. Spinosaurus aegyptiacus was the largest land carnivore to ever exist, with a length of almost 60 feet! It had a long snout lined with razor-sharp teeth, and unlike the T-rex it had long, maneuverable arms. Spinosaurus also lived on both land and in water, and was capable of swimming. Imagine that. The largest and predator (made of 20 tons of muscle) is chasing you, you jump into the water to avoid it, and it jumps right in there with you! Spinosaurus had a jaw similar to that of a modern crocodile; that is, it had immense crushing power. Even its relatives (in the Baryonyx family) were capable of all this. Spinosaurus was definitely more terrifying than the T-rex.
2. Quetzlcoatlus northropi was the largest flying animal discovered. Picture a typical Pterosaur. Now give it a wingspan of 6 humans laid end to end. Now give it a sharp beak, claws, and a fierce attitude. This monster could fly over valleys, oceans, forests, you name it! If a Quetzlcoatlus is on your tail, there's nowhere to hide. Quetzlcoatlus was even capable of long range flight. Truly, Quezlcoatlus was more terrifying than T-rex.
3. Carcharodontous saharicus was similar to T-rex, only bigger, stronger, and scarier. At 43 feet long and 15 tons, it was larger and heavier than T-rex. It was named for its jagged sharp teeth up to 8 inches long. Its 5.2-foot skull was designed for ripping up meat, and its arms were long and maneuverable. Basically, Carcharodontosaurus was a larger, angrier version of T-rex and was therefore scarier.
I took 30 mins to make an excellent rebuttal but then DDO bugged, I hate myself.
There are partly false things that Con said which is:
"almost 60 feet" & " (made of 20 tons of muscle)"
The source says the following:"12.6 to 14.3 meters (41 to 47 ft) and a body mass of 12 to 20.9 tonnes"
Therefore no way Spnosaurus is around 60 ft, but the body mass ranges between 12 to 21 tons.
T-Rex is much bigger and always huge, I will explain that later in the argument, which shows that T-Rex strength is equivalent to Megalodon, which I will show that Megalodon is seriously strong.
This is a flying animal, and the debate topic is about land animals which makes this strong dinosaur ignored and dismissed.
False(partly) things in Con's argument:
"C. saharicus ranging between 12 and 13 m (39 and 43 ft) and weight estimates between 6 and 15 metric tons." 
Again I will show that T-Rex is stronger in the argument below.
12.3 metres (40 ft) long, and was 4 metres (13 ft) tall at the hips. Mass estimates have varied widely over the years, from more than 7.2 metric tons (7.9 short tons).
"The calculations suggested that adult T. rex could have generated from 35000 to 57000 newtons of force in the back teeth, or the equivalent of three times the force estimated for a great white shark, 15 times the force of an African lion, 3 1/2 times the force of an Australian saltwater crocodile and around 7 times the estimated force for Allosaurus"
Now see this:
"However, even higher estimates were made by professor Mason B. Meers of the University of Tampa in 2003. In his study, Meers estimated a possible bite force of around 183000 to 235000 newtons or 18.3 to 23.5 metric tons; a bite force equivalent to that of the largest Megalodon shark specimens."
This shows that T-Rex is objectively far stronger than any other land animal in terms of strength, bite of force of 18.3 to 23.5, that is more than Carcharodontous saharicus, Quetzlcoatlus northropi, and Spinosaurus aegyptiacus in size.
And the bite is equivalent to the Megalodon that "Currently, most experts acknowledge that C.megalodon reached a total length of more than 16 metres (52 ft)."
And "Megalodon had a very robust dentition, and had a total of about 276 teeth in its jaws, spanning 5 rows. Paleontologists suggest that a very large megalodon had jaws over 2 metres (7 ft) across."
Even after millions of years, Tyrannosaurus Rex footprints are alive! :
Objectively Tyrannosaurus Rex, when biting any of the said dinosaurs, they will be crushed completely.
We conclude that Tyrannosaurus can crush any of said dinosaurs when such force, no one can face him.
The part of source which you quoted about the Spinosaurus was actually an incorrect earlier estimate. The source clearly says that the Spinosaurus approached 59 feet in length. An undisputable fact is that Spinosaurus is larger than T-rex, as it is without doubt the largest living land carnivore. Thus, Pro's assertion that T-rex is "much bigger" is absolutely false.
While Quetzlcoatlus did fly, it was also considered a land animal since it spent a large portion of its time on the ground. In fact, its eating, breeding, and roosting habits were all performed mostly on the ground. Therefore, it can still be considered a land animal.
I fail to see what was false about my Carcharodontosaurus facts, as they agree with what you quoted from the source.
It is true that Tyrannosaurus rex had mmense bite force, larger than any of the animals I proposed. However, this apparent advantage is contrasted with a lack of maneuverability and smaller size as compared to Carcharadontosaurus and Spinosaurus, as well as less mobility overall as compared to Quetzlcoatlus. While Tyrannosaurus was good at biting, she could not catch you as easily as any of the three animals I described.
Megalodon is a prehistoric shark. I fail to see why that is relevant to this discussion, especially considering the fact that T-rex couldn't even swim!
Tyrannosaurus might be able to crush Spinosaurus, Carcharodontosaurus, or Quetzlcoatlus, but that is on the off chance that she gets the opportunity! With the maneuverability and size advantages my animals have over Tyrannosaurus, T-rex would be ripped to shreds before he got the chance to clamp his jaws around them!
I thank you Con, please when you see me telling you that you are partly false do not take it as an offence, I wanted you to cite the full source.
Con still asserts that the source say 59ft, but that was too great, as In 2007 scientists refuted that Spinosaurus was up to 59ft "François Therrien and Donald Henderson, in a 2007 paper using scaling based on skull length, challenged previous estimates of the size of Spinosaurus, finding the length too great and the weight too small. Based on estimated skull lengths of 1.5 to 1.75 meters (4.9 to 5.7 ft), their estimates include a body length of 12.6 to 14.3 meters (41 to 47 ft) and a body mass of 12 to 20.9 tonnes (11.8 to 20.6 long tons; 13.2 to 23.0 short tons)."
While Con uses the 2005 estimates which states the following:
"Dal Sasso et al. (2005) assumed that Spinosaurus and Suchomimus had the same body proportions in relation to their skull lengths, and thereby calculated that Spinosaurus was 16 to 18 meters (52 to 59 ft) in length and 7 to 9 tonnes (6.9 to 8.9 long tons; 7.7 to 9.9 short tons) in weight."
I cited the more advanced and authentic source and Con was indeed false when he said the following:
"The part of source which you quoted about the Spinosaurus was actually an incorrect earlier estimate."
When In fact I quoted the new and advanced source.
Therefore Spinosaurus is only 1-7 ft higher than T-Rex, but T-Rex can bite and completely annihilate Spinosaurus.
I will debunk Con's rebuttal by using his own source(I know cool right!!?)
Even by the source Con provided it clearly says that following:
"Quetzalcoatlus was a pterodactyloid pterosaur known from the Late Cretaceous of North America (Maastrichtian stage) and one of the largest known flying animals of all time."
The source further says:
"In 2010, Mike Habib, a professor of biomechanics at Chatham University, and Mark Witton, a British paleontologist, undertook a further investigation into the claims of flightlessness in large pterosaurs. After factoring wingspan, body weight, and aerodynamics, a sophisticated computer program led the two researchers to conclude that Q. northropi was capable of flight "up to 80 miles an hour for 7 to 10 days at altitudes of 15,000 feet". Mike Habib further suggested a maximum flight range of 8,000 to 12,000 miles for Q. northropi."
And Con did not provide source that Quetzalcoatlus, is on land for most of the time, clearly his source disagrees with him.
I have said you are partly false because you did not quote the full measurements, Carcharodontosaurus is sometimes -1-3 ft Higher(or lower) than T-Rex, but few ft won't do anything as, T-Rex bite force is just too strong to handle.
"a lack of maneuverability and smaller size as compared to Carcharadontosaurus and Spinosaurus"
I am afraid you are false, as T-Rex was only smaller around 5 ft than both said dinosaurs, therefore science disagrees with Con as Newton's second law says: "The acceleration produced by a net force on an object is directly proportional to the magnitude of the net force, is the same direction as the net force, and is inversely proportional to the mass of the object. In other words, net force divided by mass is equal to acceleration. If you have a net force that is greater and the mass is the same, the object's acceleration will be higher. If the mass is greater and the net force is the same, the acceleration of the object will be lower."
Therefore T-Rex was faster and with his bite force he could kill said dinosaurs easily.
"Megalodon is a prehistoric shark. I fail to see why that is relevant to this discussion, especially considering the fact that T-rex couldn't even swim!"
I have stated that his bite force is equivalent to Megalodon, that is it, I did not want to include Megalodon because I am defending T-Rex in this debate.
"With the maneuverability and size advantages my animals have over Tyrannosaurus, T-rex would be ripped to shreds before he got the chance to clamp his jaws around them!"
I have already refuted that, and said that size advantages are very low(1-7 ft) therefore T-Rex has more maneuverability and can crush the said dinosaurs.
Also I do not think 3 dinosaurs against T-Rex is fair, in a fight, obviously T-Rex will lose against 3 huge dinosaurs.