Black's always been my favorite. A big part of that is the mechanics (fear and deathtouch were always ones I liked) but they also get zombies, and kill creatures. And when you're at the kitchen table, one of the suckiest things is to die to a dragon you can't do anything about. You also get awesome things that the other colors do, like hitting opponents directly (
Consume Spirit) Wrath effects (
Pestilence,
Plague Wind), drawing cards (
Promise of Power) and more.
Also, the sixth edition
Black Knight was always super awesome to me. Was one of my first cards. And
Grotesque Hybrid is also one of my early first cards, and one that I ALWAYS think of when starting magic. Along with
Soulless One, because it won me my first ever game of Magic.
Over time, I've come to be a big fan of green, as well as blue. But red has always been my least favorite color, beyond the silly chaos effects.
If you didn't notice, I've been listing a bunch of the cards that come to mind from when I first started playing Magic. I find it funny I came to be such a big fan of black when the first precon deck I bought was the Odyssey white/blue one.