you don't want a frame that flexs a lot, you want a frame with a nice compliant ride quality. That said, most current frames (particularly top end carbon) are way stiffer that needed for most of the people that buy them. I read posts by people who ride 100 miles a week or less (and don't race) and complain about too much flex and have to laugh.
anyway, any decent steel, ti, or carbon custom builder can give you what you want (or a gifted alu builder). As for stock models- I'd try to test ride some lugged or tube to tube built carbon frames with round tubes. Look and Time for instance. Javelin and Guru also make frames with amazing ride quality. Serotta as well (although we're getting into semi custom territory)
Don't even think about one of those old Vitus'. It's funny that that's the only old flex machine that people know, but that is some serious outdated stuff- and not in any good way.