Props to The Escapist's feature on the Bioware founder's top 5 games of the decade for getting me thinking about this. I've been wanting to stretch out and do something a little less serious for a while.
5. Morrowind (2002)
In all honesty, I think Oblivion has the clearly superior combat style, but Morrowind wins for shear openness of world. Oblivion's world always felt somewhat homogeneous and confined for my taste. Morrowind also has more after market mods than you can shake a stick at, while it's graphics, with the horrible exception of faces which seems to be Bethesda's kryptonite, have aged well enough that you can still stand to look at them.
4. Dragon Age (2009)
I still have some reservations in the ways in which Bioware has decided to attach a bottle rocket to the difficulty curve of their toolset, but in all other ways Dragon Age is the consummate Bioware game. A great story with segmented storyline bits to tackle at your discretion, a decent but not overbearing character customization routine, character backgrounds that have in game effects, and combat that makes you simultaneously reminisce about tabletop D&D and want to kill every member of your party personally. Add to that the full spectrum of romance options, and I find myself a wee bit giddy to be honest.
3. Dwarf Fortress (2006)
They may be @ signs, but they're your @ signs... kinda. Dwarf Fortress is probably one of the best management sims ever made, if you don't mind things actually being difficult for a change. It's obvious from the first time you play DF, and have to wait as it runs a world building simulation, that these guys are serious about good, detailed, simulation. It's not perfect, but it is a refreshing change of pace from some of the more shallow simulations, like Spore.
2. Shin Megami Tensei: Persona 3 (2006)
I think I summed up my thoughts earlier on here at SoaGCure. I don't have much to add to that, other than I think it's a little tragic that Western RPG developers seem to have completely missed what it could have taught them.
1. Portal (2007)
Portal is about as close to perfection as any game could possibly be. They refined down their core mechanics into a heavily distilled package of fun. Then in complete understanding of just how far outside the norm they were they made sure to give the players a significant tutorial. That the tutorial was entertaining in it's own right sets it apart from the many, many, terrifically boring tutorials littering gaming culture. It's only knock is that it is short, but then that could just be a byproduct of the distillation process.