Judging by all standards, the game that falls flat the most is undoubtedly Dante's Inferno. I really hate to... well... hate on games, but that particular title just didn't do it for me. While the rest wowed me in many ways, God of War 3 in particular was fantastic, DI had no such wonderful moments IMO.
Here's a basic dissection of each title and it's effect on the intended market-
Pokemon HG/SS- Pokemon... it's pokemon. The intended market will love it (except smokedhostage... but he crazy!)
Mass Effect 2- Fantastic RPG in every right, undoubtedly a GOTY contender, Bioware seal of quality... We all loved it . Saying it's the worse big game of Q1 2010 is ridiculous IMO
FF13- I'm not really a JRPG fan and I haven't played this game yet. But from what I hear on the internet, it's pretty good. Seems to have pleased FF fans in a very big way.
God of War 3- Fantastic game, no questioning there. IMO, God of War 3 is to action games as Mass Effect 2 is to RPGs. Saying it's the worse of Q1 would imply better games came out around the same time period... God must be making games then if that's the case
DI- God of War that isn't God of War 3. IMO, its lacking in all aspects compared to God of War 3... and in some aspects, quite a bit.
I think DI takes the cake for not taking the cake...IMO