If you're looking for a high skill cap tank, you're not going to find one. Pretty much all tanks have relatively low-medium skill caps, which makes them accessible to all players. If you're looking for a tank that's hard to get just right, look no further than Poppy.
Poppy is a deceptively hard champion to play correctly. Her single target charge ability, as well as her damaging Q and lockdown W makes her seem like the kind of champion you're supposed to go ham on somebody with. But if you look at her ult, it tells the true story of how Poppy should be played. Poppy's ult is meant for one thing. Okay, two things. The first is trying to snipe somebody with really low health and overestimating your damage, causing you to save them and waste your ult. The second though, is the important thing. Peeling.
Poppy is one of the greatest peeling tanks of all time. When the mid to late game comes around and your ADC is finally outputting that "Jesus christ what the f-" damage, its time to shift playstlyes. You become a guardian, and you do not stop guarding that ADC in teamfights until you are absolutely certain that (s)he is safe. Only THEN, may you join your team in glorious combat.
Use your W to stop divers/assassins from jumping on to your ADC, use your E to slam them away from your ADC into a wall where your ADC will proceed to then nail them to the wall with bullets. Use your Q to make running at the ADC hurt even more for the assailant! Keep that ADC ALIVE.