Neverwinter Nights (Bioware).

It was simply anything I wanted it to be depending on what the users themselves made out of it. Story-driven single player campaigns and modules? Check. Hack 'n slash dungeon dives? Uh huh. One shot, small group cooperative modules? You bet. MMO style persistent world servers. PvP servers, encompassing both group PvP and one on one dueling. I've had the pleasure of playing a module called Good vs. Evil that succeeds in turning the game into a tactical experience with good strategy elements as well. Another module series called Endless Nights came pretty close to making the game a convincing roguelike. The list goes on . . .

Reliably good ones. Most unique in my case could also go to the steampunk Arcanum, both for setting and complicated politics paired with strong characters and dialog. You rarely see that much subtlety in cRPGs these days.

I'd also go with Vampire: Bloodlines. God bless Troika, taken from us too soon.

Probably Fallout.

Most games, even RPGs, tend be very strict about completing content in a certain order, even if they're "open-world". I think it takes a courageous developer to make almost the entire game "optional", that is it's possible to finish Fallout very quickly without exploring hardly any of the towns and doing any of the quests. But it's left entirely up the player what you do, and what content you complete.

The mediocre game designer in my opinion is one who would think "I MUST make this content compulsory to finish the game because all this money/time/resources has been put into making it", or they think players might judge the game poorly, as being "short" if it gives them freedom to skip or do content out of order. That is the logic that gives us games like Bioshock Infinite or Heavy Rain, not games like Fallout.

Fallout 2.

Fluffers, gambling, organized crime = win.