I have created my own personal version of a counter attack squad that may not be the best but is pretty damn strong:
Ima Gun-Di; lead, self heal, defense boost (+35% Counter Chance)
Aayla Secura; self heal, stuns, 65% (+35% Counter Chance)
Kannan Jarrus; tank/taunt, self heal (when debuffed), protection recovery, 65% (+35% Counter Chance)
Count Dooku; protection recovery (with zeta), stuns/shock, 100% Counter Chance
Gar Saxon; offensive, speed down, 100% chance to assist
My team obviously has its weaknesses (Stun, shock, daze) as do all counter attack teams but load them all with a few tenacity mods (excluding Kannan), give Gar a good set of offensive mods and watch him take out any character. Kannan's counter inflicting offense down is also quite the plus considering Gar matches it with a speed down. I have found this team to be extremely annoying in Arena and pretty solid on defense considering they rely on being hit in order to counter. One last plus, anytime an enemy uses an AOE attack, certain death is imminent.