Let's all understand that most actors are jackasses in real life. Alec Baldwin and Dustin Hoffman are two incredible actors, but also total jerks. That being said, Mr. Sellers was the master of deadpan comedy. Nobody could replace him, as evidenced by Steve Martin. The Pink Panther series will forever remain my all-time favorite comedies. He could pull off anything, keep a straight face, and make everyone around him fall over laughing.
One other movie of his you don't hear much about is 'The Party'. If you love Peter Sellers and haven't seen this, your missing out. Nothing beat seeing his movies in the theatre either. What fun times those were. He is quite missed.