No game is perfect in regards to balance, there will always be some class or playstyle that reins supreme in competitive game types.
With this said, I was looking at the different classes that are offered, as well as the trees associated with them on http://tamrielfoundry.com/, and I believe the two strongest classes shall be the Storm Calling Sorcerer and the Draconic Power Dragonknight.
Particularly looking at the Draconic Power Dragonknight, which says:
"tapping into their draconic bloodline, the Dragonknight can fortify their own defenses and lock down enemies, eliminating foes quickly and brutally. "
It's a tree that focuses on the defenses of the DragonKnight, as well as, I'm assuming, single target DPS. Sounds like a Blood DK from WoW, which were pretty Overpowered in PvP scenarios back when I used to play.
We haven't seen much from the two acknowledged guilds, but Meteor looks pretty tough from the Mages Guild, coupling that with one of the above trees could add devastating damage.
The site doesn't explain the Nightblade's available trees, so who knows if there's anything out there that can out play these, we also don't know how effective the Daedric Summoning tree from the Sorcerer's side is.
Of course, everything is subject to change, and all of this is just pure speculation, but... if these two class trees end up being the dominant trees... told ya so!