As Professor Snape, actor Alan Rickman was absolutely perfect. I think a good acting work should be judge by how successfully a character convinces and is able to "transport" the spectator inside the action, and that is exactly what Rickman achieved in the Harry Potter movies. He will be missed and never forgotten.
Alan Rickman was a great actor and his role as Professor Snape was superb. His passing was sad to hear. I don't think you can point to Snape as his best acting role because he's done so many good roles. That's basically disregarding all of the other roles he has played.
Die Hard is the best Christmas movie of all time. Alan Rickman is in it, and that is his best acting role. Snape only shows up for like five minutes in the second book and the fourth book. Therefore, his minimal involvement makes it non as good as his role as Hans Gruber in Die Hard.
Alan Rickman was a trained theatre and cinema actor. While the Harry Potter films were highly popular and allowed Rickman to gain more fans, his roles in films such as Galaxy Quest, Dogma, and Love Actually were equally good, if not better, examples of his work, in my opinion. Arguably, his best work would have been his acting in live theatre, which not many people would have been able to see, unlike Harry Potter.