I love Jim Carrey.
The Truman Show is his best film and probably my favorite. It's actually one of my favorite films of all time, and I agree with the poster above that Carrey was absolutely robbed of an Oscar for his performance.
Eternal Sunshine would be a fairly close second in terms of quality (and another amazing performance from Carrey), but I have several Jim Carrey films that I would consider to be "favorites" regardless of quality -- including Ace Ventura: Pet Detective, The Mask, Liar Liar, and Dumb and Dumber (of course).
Hot Take: I also think The Cable Guy is underrated.