First off the PC has tons of great games each year, so do the Twins, and the Wii as well. Anybody who says otherwise is being a mindless fanboy. I can find at least 20 games on each system I would love to play just this year.
Frankly it comes down to what genres you like. If you are a big RTS, RPG, SIMS, flight sim, indie game fan, modder, or graphics whore 
. Many also choose PC for FPS- but not all. The HD twins are great for sports (from boxing to football), local-multiplayer sports, platforming, JRPGS, and button mashing goodness. Heck a few people prefer FPS's on them.
The Wii is great with friends, it has a great library of party games and sports games. Wii also has some great platformers, railshooters, survival horror and imports. Many consider it's FPS controls almost rivaling PC.
Why can't we all just get along?
XBL: WiiVault Wii: PM me PSN: WiiVault
PC: AMD Athlon II Quadcore 635 (OC to 4.0ghz) , ATI Radeon 5770 1GB (x2)
MacBook Pro C2D 2.8ghz, 9600m GT 512 iMac: C2D 2.0, X2600XT 256
