I prefer to play ones that are "fun" over ones that are the "most powerful". Prism Break is fun, and one of my favorites, but only when playing with someone else. Lightning Rod is also very fun to play, slinging bolts Zeus-style never gets old, and when you power up his Lightning Attack you can stand back and kill things around the corner. Terrafin is also crazy fast with his powered up Land Shark ability, and I never get tired of him humming the Jaws theme while he does it. My preferences are:
Magic: Double Trouble or Voodood
Tech: Trigger Happy
Undead: Hex
Life: Stealth Elf (only one we have opened so far)
Fire: Ignitor
Water: Gil Grunt
Air: Lightning Rod
Earth: Prism Break
These are not the most powerful, but they work well with my son's favorite as they are typically good in support roles (he likes to charge in on his Legendary Chop Chop). The game isn't so hard that you have to maximize your output to succeed, so again I go for "fun to play".
That's the thing I really like about this game, they could have easily done them as model swaps that are largely similar, but there is some real variety in how they play and I don't just mean what gates they can open. Ideally, I would like Giants or the Threequel to include four player with use of a second portal (four won't fit on one), and introduce healing and buffing/debuffing type support characters as well as ramping up the enemy count and difficulty when you play with more than one person. I would also really dig a Vita version of Giants that was more similar to consoles rather than a platformer of the 3DS version (but would include "loading" of a few characters so you don't have to carry them with you on the go).