My favorite Valar is Aulë the Smith. I like him because he’s one of the edgier good guys (or, at least, he isn’t like Manwe, who’s so good he doesn’t understand evil). Like Melkor, Aulë also wants to make things as yet unthought of, and struggles with pride, though he ultimately submits his will to Ilúvatar. Also I love that his relationship with Yavanna is sort of contentious and generally adorable. (picture by Ted Nasmith)
Honorable mention: Oromë, the only Valar who ever goes anywhere or does much of anything productive.
Melkor, later renamed Morgoth, is, predictably, my least favorite, and probably for the incident pictured here where he kills Fingolfin. He also kills Finwë, steals the Silmarils, tortures Maedhros (my main man), tortures other elves into orcs, tries to rape Luthien, psychologically tortures Húrin, etc, etc. He’s the main big bad, of course he’s going to be my least favorite. (picture by John Howe)
Honorable mention: Námo (Mandos), who is just creepy.