Assuming you’re playing a pet build (if not, you’ll probably have to ask in another thread as I only play pet builds), Conjurer’s are generally going to be the best option if you’re starting from scratch and want to do everything including the super bosses.
- Cabalists tend to get through the main campaign fastest.
- Cabalists tend to get through crucible fastest (although less reliably than Conjurers)
- Conjurers tend to have an easier time going deep in SR
- Conjurers tend to have an easier time with super bosses.
- Ritualists tend to perform worse than the other two.
@Maya has some guides for starting from scratch that you may find useful.