The Energon Pub

[Praxus Prime]

Here's another to add to my list.
Megaman Juno (from Mega Man Legends)

[sabrigami]

I would say Yunalesca and Evrai from FFX. Never have bosses required so much strategy to beat. Evrai alone took me 2 weeks to finally beat. Then using the zombie status to avoid Yunalesca's insta death attack was a challenge, especially because she tried to heal your ass at every turn