My favorite deity in D&D history is a tie between the Raven Queen. The Raven Queen is 4E's goddess of death, replacing 3.5's Nerull, who was a god of both death and undeath.
I'm a big fan of the Raven Queen because I really like the concept of a completely neutral/unaligned goddess of death. In addition, I love the raven imagery. The Raven Queen's utter hatred of the undead is probably my favorite bit about this goddess, though; while the death god with the undead cults is a classic, I really like that the Raven Queen loathes the perversion of the natural order that undeath represents.