Kimi o nosete (君をのせて)
This song is so terrfficly famous that it has actually been recorded in many piano textbooks and famously world-wide. Not one OST can contend with this song.
Also,
OP of Digimon - Butterfly
OP of Air - Tori No Uta (鳥の詩)
the ED of Clannad(haven't watched CLAS, sorry.)
P.S.: Is there any D'n'B, Techno, dubstep themed OPs?