First times are always something special. I remember thinking I was the king of the world when I rode Independence Pass back in 1982. Likewise when on a mtn bike, Kennebec pass in 1984. In retrospect, both are rather benign (although Independence tops out over 12,000ft), however the sense of accomplishment was tremendous. For the next decade and a half, mtn biking was central, and the big rides/climbs were achieved with more a sense of serenity than accomplishment - Gerogia, Imogene, Engineer, etc - just getting out into the middle of nowhere. During that time road riding just augmented summer cycling.
Moving to the (Colorado) front range changed priorities, with the bevy of road rides one could do throughout the year, but it wasn't till I traveled to Italy then France that the sheer elation of summitting a high mountain pass returned. As someone upthread said, part of the elation was/is the silent but knowing looks traded by the few cyclists suffering then succeeding atop those wonderful beasts. Furthermore, for me, it wasnt the well known climbs that offered the greatest pleasures. Ventoux, Galibier, Grappa, Fedaia etc., are all wonderful, but my fondest memories are of the Manghen's and Madeleine's.