Honestly, Outbreak.
The idea of a Resident Evil game with Co-op in the classic style is REALLY cool! The characters had different pro's and con's as well as unique abilities to each of the characters (Like Kevin's Charge Shot Ability, Alyssa's Lockpicking ability and Yoko's backpack);
The levels were really dope in that both File 1 and 2 offered levels unique to Outbreak and levels that revisit old locations from RE2, and the original levels were really cool like the Zoo and the Abandoned Hospital.
Unfortunately, it came around a time when Console Online was in its infancy and the only examples back then were PSO, Everquest and FFXI but I think that Outbreak Online in this day and age would be really cool.
TL;DR of top 3:
1) RE: Outbreak 1+2
2) RE5
3) RE7