Fire/Rad is a great combo. You will kill a lot. You will BE killed a lot. It's super effective, but you will get rekt if you get careless or reckless.
Ice/Time is much safer and easier. Weak on DPS, but you can pile on stupid amounts of soft control.
Earth/Kin and Earth/Storm are great too. Earth's -def is a big help with Kinetics, and Storm has lots of redundancy with Earth, so you can stack stuns, KDs, slows, etc.
My new fav combo is Ill/Poison. My theory was that if I couldn't buff my PA, I would debuff everything else. It REALLY shines on the Summer BB trial- plenty of time to recharge PA between AVs and it makes AVs MELT.