It's a tough call for me. The first had the best mood and story, but the second kept it going and improved on the gunplay and graphics (which still hold up on my PC). Also you get to play as Mona.
However, the third one had the best arenas and shooting I've ever played, and I enjoyed how they put cover in and made it work with the classic bullet time system.
I'm going to give it to MP3, just because this series continually got better in the game play department, even if MP2 is one of my favorite games of all time. --- We rip cell doors off, we don't ask for bail
For writing and story, the first game by a country mile. For gameplay the third game is best. --- "The problem with quotes on the internet is that you can never be sure if they're true" - Abraham Lincoln