K2SO

K2S0 is just all around good on a R1 squad. Personally I think he and Chirrut are a better combo than Baze and Chirrut, Chirrut's auto hots give him 100% taunt rate. SInce they come on damage it can only be prevented by buff immunity. Baze you can get to drop his taunt once in a while, not so with K2S0 if Chirrut is in the picture (as long as K2S0's counter is maxed and his mods/the group composition give him the tenacity he needs).

That being said there are a few other comps he really shines in as well.
Under a Luke lead he is quite effective as is due to all the tenacity luke gives.

Any group pairing him with Cassian is great.

Yoda and Rex can be good with their tenacity up buffs.

Pretty much every other comp besides the first two you have to stack mad tenacity on him for him really to be effective. You can reach pretty high levels of tenacity without too much trouble. My K2S0 never gets debuffed. Maul's daze Aoe? Spurt spurt, Doesn't daze my K2S0.

Put him in a comp with a chewie lead, zbarriss, chirrut, and Fives. His taunt will never drop off, He'll heal like mad, and almost never get debuffed. If he does it will be dispelled licketysplit. This group should be stupid tough to beat. Thanks to chewie lead he gets 49% defense when defense'd up. He himself will offense down all the enemy team. Two cleanses in chirrut and barriss. Two auto heals from them too. Two healing equalizations. 3 counter attackers in case people AoE. I want to call them the perfect defensive team.

Put him in a droid team under an hk lead and he gets two attacks for every one enemy attack. (one counter and one regular), and protects all those glass cannons. He actually is a good mesh with lobot too because the dispel could help him immensely.

Put him under a wedge lead and he does quite well as well.