Well, what class do you prefer playing as? Personally, I feel that the most useless one is to be a thief, because Vampire isn’t as helpful as it may seem. Alongside with Archers, as they do fine without the boost.
Pirates are a good choice, it’ll help you level your pirate with Sharkwave, especially if you’re going on the corsair side, since it’s tedious to level without any good mobbing.
Magicians are good for Bishops, as the flame would help for easier leveling onto 120. If you prefer the support class, this is your choice.
Warriors are also a good choice, soul driver is great for all 3 of the classes, especially paladin because of the divine/holy bonus from the skills. That’s if you want to play a Paladin.
Whichever class you want to play should be the class you pick for your UA, as it’ll receive the best benefits from any other class that you’ll make.