I play these games for the romance and emotional payoff, so happy endings feel the most satisfying. Well-written bad endings can be interesting and impactful, but I usually save them for after I've already gotten the happy one.
Bad Ending, because happy endings only make me feel happy for a short while and then I forget. If Bad endings, for some reason, are easier to remember and cherish, maybe it's because I rarely finish anything with a bad ending.