Well, strangely enough, we've only had a fighter(Greatweapon) of any variety 1 time and only for a few sessions. In the games we've played we've had:
Chaladin: levels 1-3(PHB only).
Straladin: levels 10-16(pre-DP, AV, PHB2).
Fighter(Greatweapon): level 10(pre-MP, AV, PHB2).
Swordmage(Assault): levels 3-6(post AP).
There were only one or two sessions that we had 2 defenders in the game at once(Fighter and Straladin). In the majority of the fights, the Fighter was far more effective a tank than the Straladin, though, to be fair, we were playing with rolled stats and the Fighter rolled 4 18s, a 16, and a 14.
I guess I'd put Paladin > Fighter against 1-monster fights(especially at paragon). Otherwise, it'd be:
Fighter > Paladin > Swordmage.
The majority of our 4e time has been sans-tank. Levels 1-9 of our 4-man first real campaign had the ranger and cleric splitting the tanking. Levels 3-8 of our Scales of War 3-man playthrough has had the cleric tanking as much as he can, but with 3 players, the monsters tend to be able to pick what they want...