What do you like more? Close or range attacks? Play the ones that you will have more fun.
Depends of your style, if you play moonkin you have to run away a lot of times, healing and attack again (I don't know if it's your style running and come back and fight again).
Choose unholy dk instead of blood, that char is powerfull, you don't have cons, only pros. But do you like deseases and launching infected green balls and whatever dead stuff? If you like play unholy.
I never played prot warrior, but they seems pretty nice, charge it's fun and being like a stone it's nice too.
Play the one that you think you will have more fun...it's a game. Like me I know DK it's a machine, but I like to play with feral or hunter, or rogue, cuz Im more a lonely guy doing what I want. Depends of your style and what you want to do