Mesprit has the horrible curse of being to balanced. When something is too balanced, it's hard to use it. (except for stuff like mew, celebi and Arceus, who are balanced with high stats) Uxie and Azelf both are pretty good. However, with Cresselia, there's little to no reason to use Uxie over it. With similar movepools but Uxie's HP being much lower then Cresselia, Cresselia is just more effective. Azelf has high Attack, Special Attack, AND Speed, AND a nice movepool to go with it, including Nasty Plot and Explosion. Azelf is clearly the best pixie
But I like uxie more because it looks fragile and weak but actually has tons of def. I like that