The start cause there is still about 2 and a hours hours to go
But seriously, The way the game forces you to advance based on the items you've collected, the music, the "screwattack" upgrade, the way the ice beam can be used to freeze enemies for use as step to out of reach places!!
Awesome bosses that you fear when you don't have enough ammo for!! I could go on forever about this one!!
EASILY THE BEST GAME OF ALL TIME - imho
Thank you to EGM putting me on to the game with there TOP 100 list in EGM 150 - (Which I still have now)