Nostalrius Begins - Quality wow vanilla realm (1.12)

I didn't play much of Vanilla, maybe to lvl 20. I'm thinking of rolling either a Rogue, Warlock or Shadow Priest.

I'm going to be mainly PVPing but i want a class that's fun, one that isn't just pressing 1 button all the time. Of the 3 classes, which uses the most abilities + most of all, which is the most fun?

Fun is subjective.

You will need to raid to get decent gear for pvping, and warlocks pretty much just spam shadowbolt and as a priest you will likely have to heal in raids. There are way too many rogues on this server so unless you really want to play a rogue, I'd suggest a different class. Both warlocks and shadowpriests are strong in pvp (rogues are too but good luck getting into a raiding guild as one).

The class you enjoy playing - that's the fun one.

Just about every class requires more than a few buttons in PvP (warrior being the exception, he only needs 4). So pick whatever and then struggle through gearing in raids if PvP is your end goal.
Just please don't pick undead as your race. There's way too many of those.

Rogue IMO with either Seal Fate hemo or any old daggerspec.
If brave, affliction or destro lock can arguably be fun too.

define:fun
For me it's instagibbing which might happen if you play paladin, shaman or rogue.

I've really been enjoying levelling my warlock recently, plenty of class quests to keep levelling fresh.

The answers you're going to get most are just whatever classes are popular, because it really is subjective. Maybe you press more buttons as Rogue, but maybe you don't have as much fun with that style of DPS. People always say Paladins are the most boring class to play, but I have more fun with them than any other class (especially in PvP). The bottom line is that all classes are fun, WoW would never have been as successful as it was/is if it were boring.

Just think about what role you may enjoy most, what kind of gear you like looting, if you like burst damage or sustained DPS, cloth or plate, etc. If you think your character is cool, don't bother letting other people whether or not you should be having more or less fun with them.

Condidering all aspects of the game, the most fun class is going to be something that excels in all areas of gameplay. This goes to the mage and the rogue.