If we are going on Comedy, for me, THE MASK; it fits Carrey's comedic depths.
Dumb and Dumber, along with Ace Ventura haven't aged well.
Now if we are going for the total package, The Truman Show. I say this, because it's one thing to be comfortable with what your good at but it's another to showcase to people a side people don't get to see and never expect. I tend to put more weight on Truman, as Carrey, at the time, was known just for Comedy. The Truman Show showcased how talented Jim was as a actor, and not just a comedic actor.