Because I'm poor (and cheap) it's usually whatever beer is cheapest at the local skeevy liquor store. That usually rotates between PBR, Tecate, and on the rare occasion - Modelo.
For special occasions, or around Christmas bonus time, I usually splurge on craft beer. I live right down the street (literally) from Metropolitan brewery, so that's pretty cool. If anyone is in the midwest, I highly recommend you check out New Holland Brewing Company's stuff. Their stuff is fairly easy to find in Chicago, and I visit the motherland (holland, mi) at least 4 times a year for the good stuff. Very ambitious offerings.
http://www.newhollandbrew.com/