I'd have to say Monster Hunter Freedom Unite for the wii is my top all time. There are a ton of great games from 10 years ago or so, but I don't think I've ever put as much time into any game as I did with MHFU. The feeling when you finally pull that 1% chance item after a day of grinding is sublime!
PC1: James Ramsey 2012 BDPP BGS 9.5 w/10 Auto Rainbow - Complete!
PC2: 2012 Future's Game Jumbo Patches - Complete! - But Upgrading Rodriguez and Arenado