To me the Gas Mask core has always provided a quick transition.
The Iron Fist core is quickly becoming a favorite of mine and that's just from throwing the original Warrior. I get a rounded shape but once it starts it doesn't stop at all.
I don't think you can go wrong with either, I think it comes down more to figuring out which cover will suit your needs better. I personally feel like EBI has the 3 best asymmetric cores right now with the Gas Mask, Iron Fist, and I block in the Paradox