Once Upon a Time in the West was not a success when first released (the chopped-up version). It wasn't a flop either, but it only became a smash hit when it was re-released. I worked in a cinema during the weekends in those days and they had a full house with it, day in day out, for weeks (and not only during the weekends).
About The Wild Bunch: of course it's a visually very strong film. It has a couple of other strong points, but I guess it owes its fame mainly to its beautiful blood-letting, The action sequences were imitated by other film makers (and Peckinpah himself) over and over again. But still ... it must have been one of the most discussed movies of the year, one that caused a lot of controversy (I don't really remember, I was too young when it came out and only saw it later, in the course of the Seventies, on a late-night showing) and caused a lot of controversy. In that light it still surprises me that it wasn't more successful, not when first released, not when re-released