Talisman if you have the time. ( my friend had an older edition with all the expansions, so play can take several hours.)
Open Gameplay: There are two rules in the game that stand out. First, any deal you can come up with in the game is explicitly allowed by the rules. This allows trades, bribes, and outright extortion. The second rule is that no deal is binding unless it takes place immediately. You can offer to sell a group later for money right now, and then later decide not to hold up to your end of the bargain.
That's not a boardgame, that's just D&D 4E.
(seriously, before anyone goes crazy I am just kidding)
To the subject:
I don't have it anymore but Kingmaker was one of my favorite boardgames (my game box was accidentally left in Korea when I pcs'd - long story - don't ask).
I don't know if it's even made anymore. I did a Google search and the only places I found it were on E-bay and Board Game Geek. When I was stationed in England we'd even play this at work on slow nights. The last time I saw it for sale anywhere (and where I bought it) was in the giftstore at Warwick Castle (around 1995/6). If you can find it at a decent price, this game is definitely worth the time and money.
Mmm... I'll add Galaxy Trucker to this list. Very fun and a must get as far as I'm concerned.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.