What I liked a lot about the Sonic 3 final boss music was how it synced up with the last boss in Sonic and Knuckles. During the intro he is stamping towards you as the screen shakes, then the fight starts once the music changes.
For the Ecco one, I am saving it until I actually beat that game. If it is good, then I want to keep it as a surprise.
One I liked was Death Bringer's theme in Golden Axe on Genesis. Here is the SEGA AGES version of it:
http://www.youtube.com/watch?v=k5ND_PXhBpg#I really enjoy how it starts off in a more worrying manner, slowly builds up to being more action orientated, then changes to a more hopeful tone at the latter part. It mimics how you would be playing, starting off unsure then winning around the end.
It also just sounds very good and fits the scene. Death Bringer can do all of the character's spells such as Tyris' dragon attack. Around when this game came out, that dragon sprite breathing fire over the whole screen was one of the best looking things in a video game.
