Warrior
Why I reckon warrior
-great, if not best, all round tank.
-fury dps, if geared and played right, is great for raiding
-arms is one of the most op specs for pvp of any bracket, arena, bg's, whatever. It's also awesome for solo levelling up.
Why not DK?
-niche tank. OK for raiding, problematic for newbs to the class for tanking. You hear a lot of emo about bad dk tanks, within reason. It's not at all an easy class to master. Sure you can faceroll, but you're going to be another bad dk. Juggling runes and runic power alongside cooldowns is challenging, most people don't get it right, turn out hating it and dumping the class rather than adapting. It doesn't play like any other class, completely unique.
-both frost and unholy are great at pve dps, but not the best ever. Other classes will beat unholy at single target/burst, or they'll beat frost at aoe.
-total rubbish at pvp at 80. Absolute garbage. It's ridiculous how bad this class is at it.
Why not paladin
-Seriously, the world doesnt need any more damn paladins. Most overpopulated class in WoW. Do you want to be just another 'pink dot' among the alerady zillions of them out there? Facerolling gets boring fast, too.
Of course, if you wanted to focus purely on melee dps, i'd suggest a rogue. There's a massive shortage of them and they remain the best at single target melee dps, ret pallies/dk's/warriors can do well but a rogue who knows his stuff and is appreciably geared will kick their arses any day of the week.
Also, why not drood? Bear tanks are awesome, and the class gives you so much versatility and choice.