I need help. Now I’m using as tanks Rigard-C, Boldtusk-C and Kirill, all +15-20, but I’m not impressed with results. Borill, Proteus and Kashrek-C are even worse.
I have some fully leveled 5*: Marjana, Sartana, Kadilen, Lianna, Isarnia, White Rabbit, Leonidas. Clarissa, Elkainen and one of Malosi/Vivica-C/Sif are in progress.
Whom to choose?
Out of those options I think your best bet is Elkanen tank. He’s pretty beefy and could perform well with some emblems. 
Another option is Clarissa, as Paladin is a good tank ability, and she also has an undispellable 30% defense buff against specials from her element link
A workable defense rainbow might be Isarnia Rabbit Clarissa Kadilen Marjana
Wizard Cleric Paladin Druid Rogue so no emblem overlap if I’m not mistaken
She’s slow and squeashy, I don’t have many emblems to improve her defence.
And I haven’t dark mana troop for Clarissa 
At tank, sometimes Crit is better. For very fast, that survivability might be good - she is, after all, already very fast
I didn’t say it was ideal as a defense, just something that can work. Maybe Lianna can replace Isarnia on the wing if you prefer
And what about yellow? Vivica-C, Sif or Malosi?
I’d go Sif, and maybe a Yurple front
So perhaps
Lianna Rabbit Clarissa Sif Kadilen?
In my honest opinion, from what you have fully levelled now, I would put Elkenan in Tank position. But he will need emblems
Going forward when you have finally levelled your other heroes you have a couple of options For a new Tank - either Sif or Clarissa.
Whichever heroes you choose good luck
I like Boldtusk and Rigard as tanks but Kiril is not very good in the tank position in my experience. Of the 5* you mentioned I’d say Elkanen (most effective with emblems) or Clarissa would make good tanks, I use Elkanen (without emblems) as tank from time to time in raids and he gets the job done.
It depends what level you are trying to sit at. I used BT to high platinum and Marjana as tank let me sit in diamond (barely) for a while. Elkanan can be very annoying if you don’t get him early. He can then distract you from the rest of the team who hurt you.
are you in diamond or platinum? if you’re in platinum, Lianna can be a surprisingly effective tank as everytime she goes off, she is almost certain to 1 hit kill whoever she targets. By making her tank in platinum, she often knocks out 2-3 heroes before finally going down.
In diamond, c vivica is probably your best bet at tank. She is basically a yellow kunchen without the cleanse or debuff resistance.
I feel like we need to consider the M1 Abrams in this.
For now - platinum. There’s no sense to create strong def team in pt. 3300-3500 is enough.
I have met Vivica and she didin’t perform well even vs standart rainbow, lack of disarmour resistance is crucial.
well, every tank is beatable on a consistent basis or the game wouldnt be fun. but some tanks have better synergies with flanks and are more likely to keep you in certain tiers. C Vivica flanked by Clarissa is going to be your best bet from the heroes you shared. ( my opinion).
In platinum, I suggest giving Lianna the spot, she obviously wouldnt work in diamond, but in platinum, a lot of players dont understand how to stack heroes or how to knock out lianna before she ravages their team ;).