The first two I immediately thought of were Million Dollar Mind Game and Grand Slam. Certainly, Jeopardy! is currently the most intellectual game show on American TV, so I'll include that.
If I were to stick with solely American game shows, I'd go with the following (in no particular order):
Jeopardy!
Million Dollar Mind Game
Grand Slam
Camouflage (GSN) - some of those puzzles were downright brutal to solve without almost all the letters removed, though maybe I just sucked at the game!
Dirty Rotten Cheater/The Enemy Within - The challenging part of these, and why I always said I'd be terrible at these, is because you don't want to build up too much money/get too many right answers/seem too smart, and that just goes against the gut instinct of how a trivia game works (I don't include Weakest Link in this because, even though strong players may get eliminated, that's not the point of the show)
Fun topic!
Anthony