The smoothest ride would be obtained of over the wings and optimally I'd go about 1/3rd back from the leading edge for the perfect ride. This wouldn't necessarily be the nicest ride as you'd be in the the cheap seats with a tighter pitch. You also be over various aircraft services which would make a noise when operating and then there's those sitting around you... So given the choice (and the cash), I'd sit towards the rear of First/Business/Upper/Alan Bennett Class.
PM