I find them to be smoothest up front at least in terms of side-to-side headbanging action, but the stop-and-go sensation gets really annoying. That's what a really long train will do for ya.
I've gotta go with either the final drop on Kennywood's Thunderbolt, the second drop on the lamented Texas Cyclone (it dropped alot farther than I thought it was going to!), and perhaps Raging Wolf Bobs' drop of the first turn (experienced in the back seat, right side).