Xbox 360 is the best rounded one, and the clear choice for any more hardcore gamers. It also has exclusive titles like Halo, Gears of War, and Splinter Cell. It has a great selection of games for sale on disc, as well as arcade style games for download. It is the best media center connecter and connects easily to PCs. It has many side features in addition and has excellent online multiplayer service. This will probably be the longest lasting choice, as they frequently perform online software updates and don't plan to replace the hardware until 2015, at the minimum. Also, Xbox Live is interlinked on the PC and Console, so some PC games may be played over Xbox Live
PS3 has the best graphics capabilities. The problem is that no games for the PS3 use them that well. Get the PS3 if you want a blu-ray player and want to use it for your home theater, or if you are a fan of some of its exclusives like God of War, etc. They also perform software updates, though not as frequently.
Get the wii if you are a more casual gamer or have kids. The wii has a motion controlled controller, but as a sarifice, it has a great design in games that were intended for the wii, but has bad versions of multi-platform titles and doesn't have good shooter games, racing games (without the additional wheel controller), etc. Wii has the worst graphics of the three
Prices
Xbox 360 : Standard - $249/Elite Version - $299
Playstation 3 : $299
Wii: $249 (There may be a price drop though)