http://en.wikipedia.org/wiki/Wizardry_8 if ya into old-school dungeon crawling team based RPG with modern graphic.
http://en.wikipedia.org/wiki/Deus_ex for the best overall cyberpunk/action rpg. Pick up the http://thenamelessmod.com/editors_choice/ after playing Deus Ex 4 or 5 time as it is its own game.
http://en.wikipedia.org/wiki/Neverwinter_Nights_2 for an RPG based on Dungeons & Dragons / Forgotten Realms. If ya get Neverwinter Nights 2, get the expansion, http://en.wikipedia.org/wiki/Neverwinter_Nights_2:_Mask_of_the_Betrayer; overall decent theme, choice and consequences, and decent character and http://en.wikipedia.org/wiki/Neverwinter_Nights_2:_Storm_of_Zehir as it has that "JRPG" feel to it and being able to customize your party.
http://en.wikipedia.org/wiki/Star_Wars:_Knights_of_the_Old_Republic 1 and 2 - RPG of Star Wars; great story many choices here and there. If you getting KotoR 2, try getting the Restoration content and play as it was intend by the good people of Obsidian.
You mention Diablo 2, but did you try Diablo 2 with mods like Median XL or Eastern Sun? Both overhaul offer high playability for single player mode.
I may get Wizardry 8. The rest I have already played.

Thank you Qawsed.
Fallout Brotherhood of Steel: Tactics?
I've shied away from BoS!
Suikoden II, V
FFVII
Parasite Eve
Thousand Arms
Grandia I, II, III
Skies of Arcadia
Zelda: Link to the Past.
You wont find Suikoden II cheap. V is an easy find.
Skies of Arcadia is apparently the second best RPG of all time(behind Chrono Tigger(>_>), and the greatest RPG on the Dreamcast(the original console it was made for).
Thousand Arms is sorta a date sim RPG.
I'll need to check out Suikoden, Parasite Eve and Thousand Arms. The rest I've played. Thank you!
Temple of Elemental Evil
Icewind Dale 1 and 2
Wasteland
The Witcher (it's 2007 so a bit new, but good nonetheless)
ToEE and Icewind Dale I haven't played, good shout.
The Witcher 1 and 2 I've both played and loved.
And now for some embarrassment....I find Wasteland too old and confusing (even though I have played so many old games including Arena and Daggerfall [and I LOVE DF]).
Deus Ex, obviously, and you should also try Vampire: the Masquerade - Redemption. Completely different from Bloodlines in almost every way (point and click combat, party based, etc) but really, really good. The voice acting may at first make you want to go on a berserker killing spree of carnage and destruction, but don't. You get used to it; hell I even came to like it eventually.

Deus Ex - completed. Still need to try the new one.
Redemption I forgot about!
2007 isn't a bit new, it's like yesterday.
Yeah it is!

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Thank you all. Feel free to continue posting games. At the top of my list right now are Povu's 2 mentions (DoD and Chtulu saves the world).