Boss Battles are probebly the most exhausting and memorable sequences in games.
I think I have just discovered a new favourite. The Metroid Prime series has created a whole raft of memorable boss battles form giant plants to odd one-eyed monsters made out of black goop but there is one that I have just encountered that beats them all.
The Meta-Ridley is a big winged bugger that keeps cropping up all ove rthe series but the encounter early on in Metroid Prime 3 is probably the best. After working hard to re-engage a generator Samus looks up from the top of the machinery to be confronted by the Meta-Ridley. It pounces and lands both of them in a life and death battle free-falling down a seemingly bottomless pit into the jaws of the power core. Samus has to destroy the Ridley before both bounty hunter and beast are annihilated in a disasterous collision with the core.
This battle channels the opening scene from Lord of the Rings: The Two Towers where Gandalf is battling the Balrog as they fall deep into the mines of Moria. It’s absolutely nail-biting as Samus’ Varia Suit is counting down the number of meters left before they hit the core as you try to shoot the monster down before you destry the base and the best hope to save humanity form the Space Pirates.