Generally, I like boxers, or even boxer briefs, haha. Because that's what I sleep in if I stay over at a guy's house; I'll usually ask for a pair to sleep in, or just snag some myself if I already know he won't care. T-shirts are always good, but I've got tons of t-shirts of my own. Though I'll admit, guy's t-shirts are great because they usually smell like the guy. Hoodies are good for that too, so I like borrowing them, or just wearing them when I'm over.
T-shirts. ^_^ Especially the ones with my favorite anime characters on them. For some reason, anime t-shirts for guys are so much better than the ones they make for girls. I love that my boyfriend is a huge anime junkie, too... I get to nab his shirts... x3 Lols!