World of Warcraft The Best Class for Undead
Remember that Undead derive from Humans. Humans are natural hunters, mages and well basically anything really. Paladin from uther, priest from always praying, warlock from mages wanting initiation with dark arts of magic.
Undead- warlock or rogue
Gnome- mageTauren- shaman or druidDwarf- paladin, hunter or warrior Blood elf- warlock
Nightelf- druid, rouge or priest
Orc- warriorDraenei- shaman, paladin or priest
Human- mage, warrior, paladin or priest
Worgen-rogue, Warrior or death knight
Goblin- rogue, Fire mage or warlock
Usually comparing the species with the class requirements will give you a good idea, such as gnomes are magical, therefore mages or some other magical class would suit them best. Taurens are spiritual, so shaman or another holy class would suit them best.
As you already picked the race, which is the most important decision you really have to make, the class is easy, pick whichever you like the most. A dps and heals or dps and tank. That way you can find out which one you really like and have the other just for when you need it. 2 dps spec would be over kill unless 1 is pvp.
There is no best or worst class for a race. While some races may slightly, it makes very little difference at higher levels. Like a Human Caster has a slight advantage against rogues in PvP due to increased stealth perception.
Whatever class you like best according to the race, if you like the look of a troll hunter, and like playing hunters, go with them instead of a blood elf hunter. If you like the look of an undead warlock, over an orc, go for the undead.