As much as I like the Mask, and I do think its a great film, the comics do such a better job of encapsulating how terrifying the mask can be in the wrong hands. It straight up murders people in the comics, its actually terrifying. As for his best film, that's a hard call. There is a stint of about 10 years where he was just putting out banger after banger. I really like Yes Man, though Bruce Almighty is also great.
His best role though is probably Fire Marshal Bill on In Living Color.