The first Event on this list is the Pigman Army. This is very profitable as well as the Ender Army. The hardest part is that they all swarm one player, but this opens a chance to build a rough fortress. This is THE hardest Mob related Event on Zeta, because the Event lasts until ALL of them die. The rewards from this Event include the Gold Nuggets gained from their deaths, and if the Army is beaten in 1 night/day, each player gets 10 Diamonds.
The second on the list is the Ender Army. This army is annoying because of their teleportation ability and they hit quite hard, as well as their "ragequit" when you look at them. Ender pearls can be worth different things on this server, so I can't give an estimate. The rewards from this, if each player survives the night in the open, is 5 Diamonds for each person.