Well it all depends on how well one works with the character, the gear the character has, and whether or not its a PvP or PvE kind of thing (player vs player) (player vs everything). PvE is there's no neutral territories but only horde/alliance its not disputed. PvP there are. For PvP at level 80 I wouldn't know cause I just hit lvl 80 with my priest but PvE a unholy or blood death knight would be easiest to level as long as you have the gear, money, and time to keep going.
Paladins are good characters to play as they can take alot of damage, and heal and, depending on which weapons you have can do alot of dps.
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This would all depend on what you want to do in game. Every race has a pretty decent racial ability that would benefit you in some way no matter what class you are playing.
For instance, mages would benefit from a troll's racial "Beserking" which increases attack speed/casting speed, they could also benefit from the gnomes racial "Expansive mind" which increases mana pool.
Every racial in the game has a possibility to help you in some way or another no matter what class you choose. Also, you might want to look at racial abilities that help you in PVP...some are better than others.
This is purely a personal choice. Some people will be better at one class than another. Races do not really matter.