Death
AKA: Everyone who comes face-to-face with her, from Thanos to the Silver Surfer to Captain America, addresses her respectfully as "Mistress Death."
First appearance: Death, being one of the fundamental forces of the Universe, has ever been with us. Her personification, as near as I can figure, first showed up as a Mysterious Cloaked Figure during the Captain Marvel series. Death's face -- or, at least, one of several that she can present -- was first revealed by her lover Thanos in Captain Marvel 31.
Favorite quote: Death does not deign to speak to mortals, but communicates through various minions. Even most Cosmic Entities are treated to stony silence when dealing with her. Thanos himself has never heard her voice, which has caused him no end of sorrow and anger (and given him another in a series of unattainable goals to strive toward).
Team affiliation: Death stands apart from the other Cosmic Entities like Galactus, Lord Chaos, Master Order, and Eternity, some of whom may fall to her embrace themselves one day; after all, when the Universe ends, who will be left but Death? However, one mortal has earned her regard; Thanos, the mutant Mad Titan, who to curry favor with Death has sought and obtained the power of a God several times, and has at least twice brought the Universe itself to Death's door. He miscalculated badly when he combined the power of the Infinity Gems and sought to rule by Death's side as her equal, hoping at last for a kind word from his mistress. As one of Death's mouth-pieces pointed out to him, with the power of the Gems he was not her equal but her superior, and as such it would be improper for her to address him. Such is the reward for over-achievement. In addition to Thanos, Death has met up with many other Marvel heroes and villains. As a mark of respect she personally met Mar-Vel when he crossed to the other side, but punished the villain Deadly Earnest for rejecting her by making sure he could never again touch a living thing.
Powers: She's Death, fer cryin' out loud! What d'you think her powers are?!
Favorite storyline: Death is usually handled with great respect and restraint by the Powers That Be at Marvel (as well she should be). As a result, she's been in gobs of stories any one of which, given my mood, I might, "Oh, that one's my favorite!" She stood by Thanos' side when the Avengers sought to prevent him from utilizing the power of the Cosmic Cube, destroyed by Mar-Vel with a well-placed karate chop. She made a nifty cameo appearance when Warlock confronted his evil older self, the Magus. She embraced Mar-Vel when cancer claimed him. She stared down Thanos when he returned with ultimate power after his search for the Infinity Gems, and watched in silent disapproval when the heroes of the Universe, including just about every last one from Earth and a reborn Adam Warlock to boot, came to Death's door to wrest the Infinity Gauntlet from Thanos. All that being said, however:
Least favorite storyline: Death's position in the Universe was once usurped by the Grandmaster and the other Elders of the Universe. The Elders murdered the East Coast Avengers during a softball game (don't ask) to bring the team into their new realm. This caused the West Coast Avengers to, basically, kill themselves to go rescue their East Coast comrades, who of course mis-understood their intentions, resulting in a major Avengers-versus-Avengers donnybrook in the after-world. All was eventually revealed -- the Elders real plan was to be forever banned from Death's realm as trouble-makers, making them immortal. Hard to believe that Death would let herself be taken unawares, that Thanos wouldn't come dashing to her rescue, that the other Cosmic Entities wouldn't show up and pound on those upstart Elders for her, and that the Avengers would go to such fatal lengths on the word of their arch-enemy the Collector. A decidedly unlikely storyline, even given that this is comics!