Fives is a great tank, I use him on my Light squad.
He has speed down 2 turns on his basic attack and combined fire special gives a bonus attack from an ally.
The unique skill (unlocks at lvl 4) is 30% Counter so it helps generate a lot of extra attacks. Ability upgrades raise this to 40%, 55%, don't know what it is after lvl 4 ability. I don't use him as leader though since his ability is Clone allies +15 defense and I don't run any others.
My tankiest PVP toon isn't a tank at all, at least not classed that way. I use Savage Oppress, he can take a hell of a pounding and I feel is on of the more under rated toons in the game if you use him right. I have him in a Sith heavy PVP team as leader (+30 Sith Defense skill +15 others) with Darth Maul, Dooku, Jedi Consular and Talia.
I made top 10 at lvl 38 today taking out a lvl 40 with almost 1.5K more power. Oppress was the only survivor and he duked it out at the end with Barriss. My Consular was only a 2* going in, Dooku a 3* and the others 4*. Savage Oppress definitely helps you punch above your weight.