There are four that come to mind: Kraid from Super Metroid/Metroid Zero Mission. Rhapthorne from Dragon Quest 8. Dragonlord Second Phase from Dragon Quest Builders. The epic creatures from Spore (although I didn't so much fight these as "get stood on.") In truth, I'm not sure which of these boss battles was bigger. Runner ups include Queen Metroid (Samus Returns), Oryx (Density) and Serris (Metroid Fusion, who isn't actually that big but seemed it in the battle.)