Okay, here we go:
Bands/artists:
All of those pop-punk Fueled By Ramen bands (Fall Out Boy, Panic At the Disco, etc.) - It's not that I really like them that much, it's just that I used to be obsessed with them...it's a nostalgia thing. The only band on that label that I'm not ashamed of liking is The Hush Sound, and that's because a) they're nothing like the other ones, b) they have genuine talent, c) they have some of the best lyrics I've ever heard, and d) I just adore them.
MIKA - His songs are just addictive! I dare you to listen to Grace Kelly just once.
Michael Jackson - I think everyone here has listed at least one song by him...
Individual songs:
Valley Girl - Frank Zappa & Moon Unit. Well, I love Frank Zappa...
And, perhaps my worst one (I don't think I've admitted it to anyone...)
The Sweet Escape - Gwen Stefani. Seriously, I don't know WHY I like it. The "woo-hoo, yee-hoo"s are just obnoxious, I almost never listen to it, and it's just not good. But in the rare occasion that I do hear it, I like it. It's just so damn catchy!