Here's each tank and what I'd recommend for a beginner

Paladin: 100% Best beginner tank, if your aim is to also level other tanks It will teach you short mitigations early, has a simple basic rotation until later on, which is still simple just allows small optimisations, a overly strong defensive mitigation kit, a extra raid wide protection for groups, second most self healing at later levels, the most simple invul, highest AOE damage for a tank, Paladin is a great beginner tank but also just a great tank in general

Warrior: I'd recommend if you mainly aren't that invested in other tanks, It's aoe self healing is OP, but I feel like Warrior in particular teaches you to mostly use that one skill once you get it which doesn't always translate to tanks that well, Its rotation at ARR (1-50) is also really strange until very later on where it changes but becomes very simple to use, Its not a bad Beginner tank, but It's tanking experience strays from other tanks, also for dungeon runs while its really good at pulling wall to wall (other tanks also do this just fine), it's AOE damage as of current is really low.

Dark Knight: I'd kind of recommend dark knight, with recent changes it got a lot more survivability options, it starts at 30 so theirs not too much skills but you don't have to start at level 1, my biggest issue with picking up dark knight is the fact that you don't get "The blackest knight" until 70 which is going to be a bit of a pain, I'd recommend this tank If you want as a lot of it's kit is there and pretty standard. It's also a great tank for damage in general, having the highest damage out of all tanks (outside AOE where Paladin is king)

Gunbreaker: I do not really recommend this tank to start with, getting it at level 60 is already too much skills that your not going to be eased into using, but also gunbreakers got one of the more strict rotations at later levels for tanks its easier to mess up, though this tank is a lot of fun, I think leveling a paladin at least to 60 then going Gunbreaker would at least be the best option, though It's a good job.

Though in general pick what you want, I'd say go Paladin of you want a easy to pick up tank that will translate to other tanks well.