Day 6's question is who is your favorite D&D deity. And as the old Deities & Demigods book shows, D&D deities also include a bunch of regular old mythological deities, since it included chapters on Norse mythology, Greek mythology, Celtic mythology, Babylonian mythology, Sumerian mythology, Finnish mythology, etc. It also included some fantasty pantheons that were not developed for gaming, i.e. Newhon mythos, Elric mythos, etc. Curiously, it did not feature the Greyhawk mythos, which later (in Third edition) was the default pantheon for the game. And in 4e, the gods were a mixed bag of all kinds of past deities, reassembled into a new pantheon (I should have mentioned the "points of light" default setting for 4e the other day when talking about favorite gameworlds. It was a nice one.)
However, I'm on record (see my favorite class post) as really disliking playing the cleric class. And if you're not playing a cleric, picking a god to worship isn't exactly an important task. So, I tend to like gods that can serve as interesting antagonists rather than gods that I pick for my characters to worship.
I thought about punting, and since Deities & Demigods and later the d20 Call of Cthulhu book gives rules for this, picking Hastur or Yog-Sothoth as my favorite D&D deity, but that's not really true. I also thought about punting and picking a real mythological figure, since D&D has included them, like Set or Thor or Ares or something. But that's not really true either. In reality, creating a pantheon of original gods as part of setting design has always been one of the things that I liked most about homebrewing.
Armies of the Abyss, by Green Ronin (originally a 3e product, but later bundled and re-released as a 3.5 era product called The Book of Fiends introduced the thaumaturge class, a kind of demon-lord worshipping alt.cleric. It also made the case for a pantheon of just demon-lords, and pointed out that other than good and law, your all covered. While D&D hasn't exactly been entirely forthcoming about creating a pantheon of the heads of demonic, diabolical, celestial and angelic orders as deities per se, I actually quite like that idea better than most. With a robust "court" of Outsider lords, it does, actually, make the notion of additional deities seem somewhat superfluous.
I also prefer a more open pantheon. In Greek mythology, for example, there were the 12 Olympians, but they were by far not the only gods (heck, Hades wasn't even counted amongst them, since he lived in the underworld, not on Olympus.) I also prefer room for lots of gods and goddesses and other deities, and without a formal structure that would tend to limit how many I can have, what they do, and all that. And keep in mind that in greek mythology, many of the monsters were effectively deity-level creatures too. It's easy to see Typhon or Echidna as demon-lords or deities, either one.
I guess if I truly have to pick a deity, I'm going to go with Orcus. I know, I know, he's not a deity, he's a demon-lord. But he can grant spells to clerics (at least as an option, in most editions of the game) and he's got it goin' on as an antagonist. Plus, the fat, bloated goat-headed look is a classic monster of deity-level monster. Here's a classic type of Orcus depiction, and a more comic-bookish WAR version.