Actually in the pilot it said they had been married six months at that point. I do not recall if they ever mentioned how long they dated before getting married. But yes, the answer should be Dylan and Haley, not just because of this, but because they were on/off rather than consistently dating.
Jay and Gloria's wedding was before the show even started, where as Dylan and Haley got together the first episodes of season one. By then Gloria and Jay were already married.
10. Cam is seen playing piano many times in the show. He can play both drums and piano.