I tried Kanan for a while...he's pretty bad as a tank. He is slow as dirt so odds are most of the opponents team will move before he can taunt, so he's not going to take any of their initial big attacks. To make it worse he gets no passive taunt at all so once his taunt gets dispelled, which is usually pretty quick, he's pretty useless for 3 turns. Every now and then I'd have a fight where he is an absolute god, like against Dooku...he'd just get stunned and sit there stunned at full health as Dooku would keep beating on him and healing him with debuffs...but most the time he was mediocre at best. Seems like he'd be far more useful around a Phoenix Squad than Jedi.
I switched to OB and he does a much better job of mitigating/redirecting damage. I use mind trick on his first turn and taunt on his second. Even if his taunt gets dispelled it also has the benefit of giving everyone defense up which helps keep them alive til OB can mind trick/taunt again. An added benefit is even if you don't have him geared quite as high as his your other characters you often find yourself not caring if he dies as all your other toons get a heal and buffs(and foresight from QGJ) when he does.
I wouldn't bother with Mace as a tank...I actually don't even get why he's labeled a tank...makes no sense. His kit has absolutely nothing to do with tanking.
I'd go with OB over Kanan, but if you have Aayla and R2 ready to roll I'd probably just go with them. The other added benefit of Aayla is that you'll have a second attacker so Ezra's Watch and Learn can fire off some auto crits...it would be pretty beastly to have Aayla attack, auto crit, stun, possibly call someone to assist, and Ezra also follow up with 1-2 auto crits of his own.