PC only:
RPG of the Decade: Jagged Alliance 2
Runner up: Knights of the Chalice
Third: Silent Storm/SS: Sentinels
Hmm that might get me in trouble... I'll disqualify JA2 and SS by labeling them as "tactics" games. I'll also disqualify roguelikes as their own genre.
RPG of the Decade: Knights of the Chalice
Excellent AI, excellent encounter design, classic D&D character development, extremely focused with little wasted time, kept even the incredibly bitter and cynical adult version of myself which hates everything mesmerized throughout. Had some dungeons where I was really struggling to use/conserve all my resources to survive, which is sorely needed in CRPGs to make the dungeons have at least a little bit of tension.
Although to be fair, the early game was probably a lot more fun for me than it will be for most people as the campfire in the Orc fort was originally placed in a hard to notice area and I missed it and had to wait until after the boss to rest, making the dungeon much more challenging and enjoyable than it would otherwise have been. Also eventually you figure out crafting ruins the resource conservation/survival element, but the battles can still carry it, these are some of the best battles ever put into a CRPG.
Runner up: Baldur's Gate 2
Some ridiculously high number of hours of B - C+ level D&D combat and adventuring, plus nice character development. Sure, the combat system isn't exceedingly tactical, but I can name 50 RPGs with worse combat, this combat was also a far cry from say KOTR, you still had to use tactics for some encounters and I imagine most people died a bit first time through. They say quantity has a quality of it's own, I'm not sure that's always true, as 100 hours of D level content sounds like hell to me. But this game had both decent quality and ridiculous quantity, so it was great back then although my patience for it would be lower today I'm sure.
Third: Icewind Dale 1
Better atmosphere than BG2, full party was a big plus too, but ultimately not as memorable encounters and suffered from certain problems like NPC groups getting split apart on pulls.
A proper P&P system with thousands of hours of playtesting adapted to a CRPG just has a much better chance of having good mechanics, compared to something made up from whole cloth by non specialists in a couple of months. (Sir Tech did ok with making up systems though.)
Honorable Mention: Odium
Nice combat system, bonus points for the extremely limited resources early on, creating a need to struggle to conserve resources, not much wasted time. Sadly limited resources do not last.
DISHONORABLE mention (bad)
All Action "RPGs" and those people who put them on RPG of the decade lists.
Arcanum - due to massive gameplay and character development failures, we've been over this one ad nauseum
Wizardry 8 - Speed patch made it almost playable, awesome character development, but ruined by multiple 15- 30 min combat with some randomly generated alligators or something in order to get anywhere, Tried it twice, got bored twice.
TOEE - It's like KOTC with awful encounter design, the most boring town in any RPG, and an extremely poor difficulty curve. Tried it twice, got bored twice. What a waste, they make that awesome engine then have like one fun battle in the first 5 hours (the main moat house room).
Pool of Radiance 2 - Speed patch made it almost playable, but combat is almost repetitive and boring as Wizardry 8, although at least they're not random battles. Better battles than Wizardry 8, but almost no character development at all. Tried it twice, got bored twice.
Vault Dweller said:
I don't think that there is a single player who would prefer bad/mediocre graphics to good ones, if everything else is equal.
I would prefer mediocre quality 2d sprites to high quality 3d graphics, both at an aesthetic level and because all else cannot be even, 2d with mediocre tiles means animations can be vastly quicker and the interface can be better with no need to worry about the camera angle.
(in general, not an AOD related comment)