The final boss of ratchet and clank 3: Dr. Nefarious.
My view will be done from fighting him with little to no armor.
Now, Dr. Nefarious is an amazing final boss. He pulls no punches, and with no armor he can easily 1 hit kill you (2 if you're lucky).
He's very fast paced, and tests you on all the dodging and quick selecting skills you've been working on up to the games conclusion. Even if you just cheese him with good armor, the shield charger, and the overpowered weapons in the game he'll still give one valiant fight back. He's easily one of my favorite bosses of all time.

Final boss of Dark Souls: Gwyn
Now, if you give this guy a completely fair fight, he is a nightmare. He's the most aggressive enemy I've ever fought, and in a game like Dark Souls, that says a LOT. He hits like a truck, he hits as fast as a tiger. He, like Dr. Nefarious, tests everything you've done in the entire game. If you have a low stability shield, have fun dodging six hits in a row all with less than a second delay in between (no, trust me, it's fun ).
Almost all the bosses in dark souls could be talked about with the same praise, but Gwyn encompasses nearly everything the other bosses threw at you, and being an absolute nightmare for both the over-prepared and the unprepared. I love this guy, and next time you fight him, try it without parries OR a shield (with light weight roll or Dark Wood Ring of course), it's a totally different experience.