My current level 30 mage (uses only fire spells and atronachs and wears only mage robes) is and has been much more powerful than warriors I've played. Granted, my warriors used one-handed weapons and light armor, so they weren't the toughest they could be... but the difference is so vast that I still think my mage could outpower two-handed, heavy armored warriors. My mage has been able to defeat enemies easier and at earlier levels than my warriors have. He's taken on a herd of three mammoths and two giants with my mage, and killed them without getting a scratch—something I never could have done with a warrior. I've also taken out entire Forsworn camps all by myself.
That being said, I think any class could be the "best" depending on A. your own natural preference, and B. what you do with it. I'm naturally a stealthy player, so it's much easier for me to kick ass with a rogue who can one-shot an enemy with a dagger, than with a warrior who goes barrelling into battle with a warhammer. But I know people who hate taking the sneaky route, so they do much better with a warrior. It's more about the player than the class, really.