Depends on what you’re trying to do and what gear you have.
Campaign:
Cabalist: Fastest and is probably the easiest up through Elite but skeletons will die a lot on Ultimate if you’re underequipped.
Conjurer: Not quite as fast as Cabalists but perform better late game if you’re lacking equipment and are a bit easier to play overall.
Ritualist: Not a lot going for them compared to Cabalist or Conjurer. They can do okay but are noticeably less effective late game.
Solo Class Shaman: Capable but it’ll be slow if you’re lacking items, and you end up having to take some non-pet skills because you have too many points.
Solo Class Occultist: Similar to the above.
Solo Class Necro: I wouldn’t try this. The extreme lack of support skills will make late game very slow and difficult.
Crucible
Conjurer: By far the most flexible and robust. Tons of builds can do it easily.
Cabalist: The best Cabalist build can compete with the best Conjurer builds, but otherwise there is far less flexibility and reliability and requires better piloting.
Solo Class Shaman: Capable if you have the right items, but a lot slower than the Conjurer.
Solo Class Occultist: Capable if you have the right items, but a lot slower than the Shaman.
Ritualist: It’s possible, but very unreliable and hard to play.