Never had any grey outs on Swarm, but maybe that's because i've ridden it so many times and I can anticipate all the elements? I agree with most others though that the closest moment is the helix near the water.
The section after the mid-course break run Goliath at SFMM was my first experience of greying out and it left me utterly terrified. Fortunately i've learned to enjoy it now though and have experienced grey outs on both Manta at Seaworld Orlando on the pretzel loop (surprisingly not on Tatsu's) and pretty much the entirety of Black Mamba on the back row on my latest trip to Phantasialand last week (could be to do with the fact it was the morning of a super hot day and I had not eaten much).