Let’s go Eevee
if i could say both, I would. . . But i love <3 eevee
The Let's Go games don't seem to be very popular, So it won't be very easy to get into a battle online.
Splatoon 2 is by FAR better. It is such a fun game, And it's a classic. You can do so much, Like OctoExpansion, Try new guns, And a fun story mode. It honestly depends on your preference though. Splatoon 2 is a lot more fun in my (altered for some) opinion though.
Unless it is really, Really, Popular like detective pickachu, I honestly do not bother with the side pokemon games, If there is an core one that comes out, Like for the 8th gen, I would consider buying. Also, It connecting with pokemon go is a huge turn-down for me.