This one seemed pretty easy for me... These three all have a lot of meaning for me.
1. Shabondama - again, perhaps my favorite MM song/single ever. This song was just full of awesomeness. Resonant Blue rose up my list after I heard it in concert, but this has always been at the top.
2. Happy Summer Wedding - First MM song I ever heard, though I had no clue what it was at the time. Love the c/w also. Again an awesome single.
3. Go Girl! ~Koi no Victory~ - I think this song makes me the happiest of almost any. Lot of memories there.