Aberrations, Outsiders, Humans, and Undead.
I like a Lovecraftian, horror-themed campaign, with lot's of tentacled monstrosities and demons, undead hordes, and insane human cultists. I also like a highly political, Dune flavored "plans within plans" element, as well, and so human opponents are a big part of my games. You're far more likely to encounter a 26th level Human Fighter in my world than a Great Wyrm Red Dragon.
However, my favorite custom monster I've ever made, and the one I'm most proud of, was the Fire Elemental Monks. I had the most amazing visual for them, and I hope I was able to explain it properly to the players; flickering wisps of flame that would dance around and lash out with licks of fire, and then dance out of the way of incoming attacks. Dang, they would be beautiful to see in a fight.