Claptrap arena chest questions

What is the loot difference between the slices of cake and what are the best chests availabe. So far I’ve been farming the purple chests on 6 slices of cake and frankly im getting nothing so im kind of curious.

best chests are glitch, worst chests are grey. higher difficulty = higher quality loot. lvl 9 and glitch chests are incredibly common legendarys (By which i mean, i found a fragnum, maggie and shooterang in one chest)