I could talk forever about fonts.. and I still suck at typography.
Anyhow, after being in design school, I tend to avoid most of Dafont's offerings, as the vast majority are made by amateurs and have quality issues like bad kerning and proportions. However, there are a few gold nuggets there, basically all of which get showcased on Smashing Magazine's website. If Smashing Magazine has featured a Dafont font, then it's a good font.
Otherwise, I tend towards the classics and other fine fonts (because I'm a snob like that).
My favorite sans serifs are Gotham and Futura.
Gotham is gorgeous all around, in all its weights. It's the font largely used in the Obama campaign. I've had it since before the Obama campaign though, so nyah.
Futura is a classic. It's been around since the 1920s, and was the inspiration for Century Gothic. Both these fonts are clean, modern, and awesome. I avoid Helvetica because it's so obscenely overused. Same with Arial.
For serifs I use either High Tower or Jenson Pro. Usually Jenson because I have all the weights and styles. Both these fonts are stately and elegant, and aren't boring like Times New Roman. However, these don't have convenient Wikipedia articles.
But yeah, if any of you are actually serious about graphic design, I highly recommend getting to know your fonts.