[SixOfSpades]

On the whole, the best Fighter is a Fighter/Mage, because of his ability to cast things like Stoneskin, Mirror Image, Prot. from Magical Weapons, and various spell protections right before a big fight. A Fighter/Mage with those spells up would absolutely WASTE a straight Warrior in a melee brawl, no questions asked.
There are drawbacks, though. If you Dual-class a Fighter->Mage, you won't get the cool Warrior abilities like Greater Whirlwind. And if you Multiclass, it'll take a LOT longer to get your Saving Throws down to the point where you're immune to most spells. Also, you won't get the Berserker's enhanced AC and immunity to Imprisonment, or the Barbarian's greater HP, resistances, & immunity to Backstab, or the Cavalier's Saving Throw bonuses and resistances. It's impossible to decide....but you'll have to anyway.