My first thought in looking at DnD was to make a paladin - they're the best tanks in WoW, where I play a prot paladin, and 4E seems to have decided they're one of the main tank options now for DnD, in addition to introducing 'tanking' to table top play with the aggro powers...
But, if I were purely looking at it from the PoV of min/maxing it, which one would perform best over time, through the levels, at being able to force threat their way and 'defend' better.
Versus single target mobs?
Versus multi target mobs?
On the one hand the fighter's core aggro ability, Combat Challenge, effects every target hit in a round - that means that if the fighter does something like a cleave or a Sweeping Blow they 'gain threat' on a lot of mobs at once.
On the other hand the paladins does damage to anyone who resists it, making it very costly to not 'aggro' a paladin that has marked you with divine challenge. However Divine challenge(*) can only mark one mob at a time. The paladin does have abilities that can multi mark, like piercing smite, but those only trigger the usual marking perk of to-hit penalties.
(*) Divine Challenge reads very strange. It says I mark the target, but it also says "Close burst 5". And then it says "one creature in the burst."
Why exactly is it a burst if it only effects one creature?