I mean biggest as in size. Some of you might remember fighting Tequatl in that picture up there, but look at this.
https://wiki.guildwars2.com/images/2/23/Elder_Dragons_size_comparison.png
You see that tiny little fly on the right? That’s Tequatl. You see that big purple and black dragon close to the right? That’s Kralkatorrik. You fight him in a story mission in Guild Wars 2, and he’s so big that the newest zone is on his back.
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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.)