With the right build and moveset, any can be great. Sunbreaker can have bottomless melee and cauterize self heal, and a potent Super. Defender has amazing protection, plus massive orb generation capacity. And Striker is wonderfully simple for a beginner to use, but remains very strong in expert hands as well: it has good grenades, it can counter Supers with its own, and its shoulder charge is potentially very effective.

I change according to how I feel. I'm usually an aggressive player so Sunbreaker or Striker. If I feel like supporting and producing tons of orbs, it'll be Defender. Probably for me, I main Sunbreaker. The shield from the sunspot is laughable so just keep moving (and Immolation Fists are a must). The Suncharge build is pretty interesting too.

I vote Defender but, I'm biased.