Ig88 no doubt! Hk47 is really good for his leader ability. The leader ability stays the same wether he is 5* or 7*. The IG's are for the heavy damage, but they are squishy. Starring them up gives them health and damage.
The only problem with a 4* HK47 is people will notice him in arena and maybe target you.
I run the cookiecutter droid team on my second account 4* HK47, 7* IG88, 6* Ig86, 5* Poggle and 4* Poe and reached rank 1 today. I'm level 63 and beat level 70 players with 6k power difference. Was pretty surprised myself.