I hope this one will be conversation-heavy! :p
http://www.gamespot.com/news/the-simpsons-rpg-has-been-considered-6412592

Give Alpha Protocol a try. I swear you'll be pleasantly surprised. It masquerades as a stealth action game, but there's a lot of dialogue and decisionmaking involved.

Then there's The Witcher games, which are very story-orientated.

If you're willing to track down a 20 year old game, BloodNet.

Yes! This. Probably one of the best use of conversation options in any game. Also the only game which is truly non-linear and still keeps up a good storyline.

Albion has alot of dialogue, tons of it. Lots of lore, story and stuff. Similar to Avatar, the movie that was based on it.

NPC dialogue? Ultima series comes to mind. Ultima 7 for sure. Ultima 6 did pretty well in this department as well, IIRC. Ultima 5 Lazarus (Dungeon Siege mod) also.