This is simply for leveling sake;
Cleric - Justicar/Shaman (once you're higher level you only put 18 points into Justicar to get your Even Justice cleave to hit 5 targets, rest into Shaman for the Dmg)
Mage - Warlock/Chloromancer - this build kinda goes both ways, the way the souls need to be kept up is slightly wierd but this build was really fun while leveling and next to the Justicar/Shaman build for Clerics damn near unkillable while pulling 3-5 mobs and AoEing them down.
Rogue - Bard/Ranger if you wanna have no downtime/ a pet to tank, Assassination if you want to kill shit extremely fast but die easily if you get more than 2 mobs on you
Warrior - Riftblade/Reaver with a semi balanced between the two with more into Riftblade. You use a 1H/Shield with this build, and this build is extremely powerful at killing mass groups of mobs when you're 36+