It's possible that I'm lying to myself, but one of the smoothest riding bikes I've ever ridden (and still own) is a mid-level aluminum Colnago. It also feels the softest (laterally) compared to most of the frames I've ridden/raced. Sure, tires, wheels, bars, etc. make a difference, but when I'm swapping the same wheels between bikes, have the cockpits set up with the same parts, and am using the same saddles, attributing noticeable differences in feel to those components alone gets harder believe. Geometry differences and fork compliance differences I would include as probable significant factors. Years ago, I got a new team frame (Felt - big tube, aluminum). I took all the parts off the Colnago to build the new bike. The difference in feel was vivid immediately - harsher, stiffer, twitchier.