Whatever you like. Think about what you want to do at level 80.

Just don't take 2 tanks or 2 healers and you'll be fine.

If you really have no clue go with a hybrid class so you got options left. A paladin or druid, so can choose to tank at lvl 80 can be handy.

Or just 2 mages can be plenty of fun, just less different options of what to do at level 80.