Definitely devils. Demons are dull and overused, and most recent WotC books and/or Dragon and Dungeon articles focus on demons. Although there are infinite varieties, they are in danger of becoming the new drow (used in everything). I find demons too direct, brutal, and in your face- they are scary physical adversaries, but not much past that.
Daemons are ok, but seem somewhat out of place. Demons are pure malice and destruction on an animal or emotional level, devils are the diabolical plotting evil of the intellectual, and daemons are....? Well, not really sure. I never liked the whole Blood War concept, so in my games I don't use it. And without the Blood War, daemons as written have no real role. I really like Green Ronin's take on them in the Book of Fiends- the representations of deadly sins, but thats not very D&D.
I love me some devils though. They are dangerous and evil in a way demons and daemons can't be- they lure you into an iron-clad trap, take everything you have, then ask for more. They are the insidious, plotting, eternal evil that uses a person's strengths and weaknesses against them. They are pretty hard to DM well, but when it is done, players sit there with their mouths hanging open at how horrible and subtle evil can be. Plus you've got Asmodeus, Mephistopheles, Moloche, Bael, Belial, Mammon, and Dispater which are so much more interesting and deep than Orcus, Grazzt or Demogorgon.