I agree with Schwarzkoph- first time I rode the Greased Lightning at Astroworld the loop surprised me. It doesn't look intimidating (todays standards) but packs a punch.
Also...the old Arrow shuttles. At Riverside (now SFNE) they had a ride called the Black Widow. SFNE ripped it out when they took the park over. It was a big deal back in the day as it was New Englands only looping coaster. I always remember the "blood rushing to the head" feeling on that thing.
well, i have only ridden one Schwarzkoph and that was when i was very young so i don't remember very clearly. however, the pretzel loop on PKD's oldarrow standby, Anaconda, always makes me come close to blacking out. if you think about it, those two loops in a row are a pretty long period of sustained g-forces. they're up high but they've got a tight radius. it's really the only thrilling part of that whole ride.
Usually the double loops like on SFOT's Shockwave get me. I greyed out on that back in the 80's. Not sure how those loops have held out 20 years later but they were intense back then. SFOG's Mindbender has my fav intense loops of all time.
I've been on SUF and while it is intense, I don't think it's as bad as X-Flight's loop. Both are pretty much equal on my scale, but with X-Flight's loops, it's near impossible for most to even breath when that thing is pulling out of it.