For favorite outsider there are a lot of options, Glabrezu, Tanari'i, Balor...but the one I like most is the most Ravenloftian of Outsiders, The Night Hag.
That's right, the simple Night Hag. Her primary occupation is to harvest soul larvae, which are the soul husks of the selfish individuals who die and descend into the lower planes, and then sells the them to demons and devils. They also trick men into mating with them by appearing as beautiful women in their dreams, and then after the event they slay and eat the man, black-widow style. But most importantly, in the game Planescape: Torment, it was a Night Hag, Ravel Puzzlewell, who gave the Nameless One his immortality (for a pretty hefty price) and allowed him to go on his crazy adventures.