Class / Spec to enjoy

Hey! I come from a gaming background of very competitive, high action gaming. I'm looking to either play a DPS or Healer, and preferably one with a very fun, high skill cap. I get that fun is subjective, but skill cap shouldn't be. I originally posted to r/WoW, but they're really against new players over there. Does the game get harder the higher level you get? My highest level is 18, a rogue, and it seems simple still (Although I understand lvl 18 is low compared to end-game, I'm still trying to consider buying the game). Also, which healing classes have the highest difficulty, and which melee dps classes feel most satisfying? From writing this, Paladin seems to match both healer and dps, however he was my first character about 10 years back, when I had the game. I've since lost access to the account, and vanilla might be too different from current WoW, but I don't remember struggling to keep up with abilities (granted I was terrible at gaming)