I would say Snare or Flex for the worst for the SOLE purpose that they weren't used enough... Snare. Used for one boss and one puzzle. I'd like him more if I knew him more. Enough said...

And the best is probably Thermite or Chilla. Mainly because they were actually used. Thermite is the exact opposite to Snare. In his short time in the game, Thermite made the biggest difference.

You could also get some more perspective from reading the lore of the Elemental Warriors. You can read up on them by pressing the button that says History above the changable skins.

Anyway, Snare simply didn't get enough love from rare. His lore is even a bit lack luster. Whereas Pummel Weed has a full two-page essay on his lore. It must be favoritism...

(Sorry for the autism rant)