Those are two very different questions. Stam is winning the damage race at the moment. I havent seen much on stam warden, but the other 4 classes are all breaking 40k on target dummies.
The best build for leveling is probably a magic sorc. Surge combined with a lighting staff is hard to beat on trash mobs if you are just trying to grind XP. mSorcs were in first place last patch for damage, but are now middle of the magic pack. They are still very strong and is probably the best class to break into end-game with.
Stamina Warden because it has great damage and very strong healing options. Putting Lotus (and using it) on your bar means you will be constantly healing for doing the thing you want to do while leveling anyway: attacking. Plus it has very cheap ultimates which are very helpful when leveling. A lot of the time with other classes while leveling you can sit with your ultimate at full until you get in over your head, get to a quest boss, or you get gutsy and pull a large amount. A warden has an AoE, single target (and personal off tank), or heal ult available every 2-3 packs. While I agree other classes may have better damage output, the peace of mind, reliable damage, and versatility of the warden, I think, is much better.