It really depends on where I am, but Mascot is always on the safer side. You can beat any enemy in the game with three Mascots and there'll be only a few that actually cause you problems. Although admittedly, Berserker and Dark Knights can also be real beasts. Berserker dodges all attacks and Dark Knights can deal lots of damage real fast.

So best dress sphere is definitely Mascot for all three, best grid is either Higher Power or The End (really doesn't matter which you pick since the Limit Breakers are present on both). There's just nothing that can really compete with that.