My favorite of the two is probably OoT. Due to the EPICness and best dungeons in the series. OoT is a flawless game (but not my favorite zelda game :P) MM is everything I could want in an overworld and side quests, but I focus on dungeons a little more :P and, I suck timed.....
If after OoT...
Majora's Mask hands down.
It just is richer and it feels like your actually DOING something in the game like in OoT you can spend eternity just walking around while in MM its more proritized.