Which Dungeon Defenders DLC is best?

The classes if you want to play something different, the mission packs if you want more missions.
To be honest, none of them are really essential, it's just a matter of taste.

The DLCs are all pretty bad. The business-model with Dungeon Defenders is get out the game cheap in Indie Bundles/Sales and then make the money wiht the 78 DLCs...
If you really like the Game you can take a look at the Hero DLCs, but the Rest is pretty much a waste of money.

I just bought all of the classes, because I want to play as different characters.

None of them are essential, though the eternia shards pack is fun if you already beat the game with a high level character.

Eternia shard pack&Series EV if you want to try solo end game content for equipment and what not.