I personally think 
takes the most skill but honestly this is like a really biased opinion since I 1 trick her currently :P.
I think that one part of tanks in generally that is very under appreciated as a sense of skill is the awareness in a teamfight. Most people see assassins as needing the most I think, since they need to know the right time to go in and kill the carry. Tanks, on the other hand, have a different way of showing this skill. Poppy mainly plays around picks. However, when they dive on your carries, you need to know what target to go on. Are you going to peal or will you dive to tank vs the enemy carries and potentially kill them? What target should I ult as poppy to turn around the fight? What time is a good point to use it? Other tanks are similar but I think most of them mainly try to engage or get picks like sej/zac and other champs. They also need to know what target to go on to help the team in the best way possible.
Some of the easiest champions that are playable are tanks, like garen, malphite, and maokai (some of it is debatable but they are pretty easy to play) but they also need to be aware of what is happening in a fight and make a quick decision that will benefit the team the most. It's true that tanks have lower mechanical skill level than other champions but some if not most of them require more awareness in a teamfight than them.
Just saying I do not play bruisers currently, although I will try to get some in champion pool soon, but most of them just try to dive the backline and not peal since they don't have the durability and the cc that tanks provide.
tldr: I think poppy takes the most skill. I also think that tanks have this skill of awareness for a teamfight which a lot of people seem to forget and this makes them harder than they would appear.
