Try Johanna, Muradin and Arthas.
Johanna is an easy tank. I mean, it’s hard be bad at her, but also hard to be exceptionally good.
Muradin has an instant escape, sustain from level 13 and an oh s ult, so you can stay as long as you have to then jump away, let your trait heals you up and do it over and over again.
Arthas: Insane peel, kill setup and great sustain after level 10.
Personally I have the most issues with tank players because they don’t know what is their actual job.
Provide vision to your team! It’s a really important and underrated kind of help for your team. There are assassins with higher cast range than vision range and they can kill something if you give them vision of the target area or the target itself. Don’t forget to scout the bushes when you rotate together so you can cover your teammates when you are ambushed. Some higher ranked players cast skill shots into the bushes for you, so when they are clear you can go in there to further extend the safe area where you can move or sneak closer to the enemy ie. when you are about to ambush them on their camp or boss (if they are dumb enough to do it when you are alive)
Use body block! Clearly Arthas is the easiest tank to do it because of the permanent slow, but others can do it as well. Either block your opponent’s path or try to get between an enemy and its target.
Zone! Certain areas should be covered and as a cc tank the enemy takes high risk if they decide to walk into that area. Cover the (channelable) objective from a bush close to the objective, from the direction you expect the opponents coming. In this way you provide vision, you can figure out how are they planning to attack you and you can counter it. Imagine they are on their way to the objective and a mass root/stun comes out of the bush. Block skill shots when you can, so your teammates can get the objective without a fight.
Protect, setup! Obviously you do this 2 things in team figth and a general tank issue is rushing forth like a horse with blinders, ignoring everything. You don’t have to go further from your teammates than range 4. If you go forth you are vulnerable and open up a big space between you and your teammates and your opponents can either isolate you or killing your backline you left unprotected.
The range ~4 is important for several things. You shouldn’t take damage in vain. Whoever attack you should pay the price for it, and if you are close to your assassins they can punish them. When the opponent try to initiate a kill on your teammate, you just simply take a step back and use slow/root/stun/blind whatever you have and appropriate for the situation. It’s pretty bad experience for an enemy who can’t reach your backline but gets close enough to get ccd and die especially if your assassins trust you and immediately punish the caught opponents. When you initiate a kill they also have to be close to you. I guess everyone saw that type of ETC who used 5men mosh but so far from the team that no one could have attacked the opponents.
Do not chase opponents when your teammates are under attack unless you clearly know that they can deal with the situation without you and you can deal with your target without them.