Jeopardy is a given. The people on this forum are all super smart and full of trivial knowledge, so Jeopardy is where it's at for us.
But what other game shows do you like/watch? I like trivia, so shows with trivia as an element are my favorites. Also shows that require some form of player strategy.
I like Cash Cab, Catch 21, and The Millionaire. Cash Cab asks a lot of questions and keeps you thinking. Catch 21 combines trivia with strategy and the luck of the cards. The Millionaire is slow and tedious at times and the early questions are silly gimme's, but all and all, it can be fun if you know a lot of fairly obscure facts and the players are willing to take a chance or two.