The ones with Rachel, and not necessarily the ones about their relationship. There are very great moments between that are fantastic when they are just friends like: when they go to the store to buy Ross’ couch (a little before the “pivot” scene); when they try to embarrassed the other at the plane going to Vegas; when Rachel drives Monica’s porsche and Ross goes with her; when they sing “Baby Got Back”, etc.
But obviously, as a Ross and Rachel fan, my favorite moments between them are the ones related to their relationship.
And I really believe when Ross was with Rachel, it were the scene where he most looked happy and had fun more.