what? Echoes has the biggest one which was quadraxis. I think metroid prime was bigger but the battle really didn't make that obvious, Quadraxis battle was just epic.
I'll be honest i don't remember who that is. I remember more from Prime 1 since i've played that twice, where as Echoes was played once and Corruption is still fresh in my memory.
There are only two bosses in Echoes that i remember:
1. That giant Bee thing that flew around that poisonous arena and shot smaller lil bee things at you (epic battle)
2. That annoying boss you fought in morph ball mode.
The second was obviously pretty small (so were the Ing in general) and the first, though big, was not as big as some of the memorable boss fights from Prime such as:
giant weed monster thing (no stogi, not that kind), or big rolling rock thing, etc etc

EDIT: You're right the battle against
Quadraxis WAS epic. The second i saw this pic it all came rushing back to me. Wow.