In no particular order -

Being There
Rain Man
The Party
On Golden Pond
The Jerk
Blazing Saddles
Second Hand Lions
The Producers
A Shot in the Dark
Anger Management
The Pink Panther
Midnight Cowboy
Revenge of the Pink Panther
Donnie Brasco
Scent of a Woman
Guess Who's Coming to Dinner
The Pink Panther Strikes Again
The Fugitive
Forrest Gump
The Insider
Return of the Pink Panther
Hero

OK - so there's 22 instead of 20. Luckily I was able to stop at 22 ... I could go on with probably 20 more, and there still wouldn't be room for some of the films that most others have listed - like Goodfellas, The Godfather trilogy, Saving Private Ryan, Casino, Citizen Kane, Kill Bill, Pulp Fiction, etc. Different strokes and all that jazz ya know. You can probably tell from my list that I'm a huge fan of Peter Sellers since I've listed 7 of his films in my top 20 (22).