My love of the Wii is no secret
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but with every bit of seriousness in me Versapak hit a great point....there's just not a lot of great single player games out for it.
I pretty much only play single player games on the Wii and I still love it

Lego Indiana Jones, Super Mario Galaxy, Super Paper Mario, Okami, Res. Evil 4, Res. Evil Umbrella Cron., Metroid, No More Heroes, Zack & Wiki, Zelda, Guitar Hero 3, Bully, Trama Center, The Godfather, Excite Truck, Elebits, Fire Emblem, Marvel: Ultimate Alliance, Tiger Woods PGA, Mortal Kombat, Scarface, Tomb Raider, several Dragon Ball games, Medal of Honor, Metal Slug, and Battalian Wars.
All of which have gotten good to great reviews and all of which are mainly single players games (granted several also have mutliplayer). I'll admit there hasn't been much in the past couple of months and with GTA IV on the 360 and Mass Effect and Age of Conan on the PC, the wii hasn't gotten much play lately, and looking at the upcoming release list for the Wii this may not change anytime soon. Usually every system has a big summer drought of games, but it looks like the Wii is the only one this year.