Most memorable Boss Battle~

I'd say Tower Knight from Demon's Souls.

But Poseidon or Hades from GoW 3 were epic also. So was the Argus Robots from Vanquish.

Resident Evil 4 had some excellent ones.

From this gen:

Jubileus, Father Balder and Umbra Witch Jeanne - Bayonetta

Liquid Ocealot - MGS4

GLADOS - Portal

From all previous gens:

Lavos - Chrono Trigger (the first final boss ever in a JRPG that i faced)

Ganon - OoT (My favorite boss from all time)

I know there is going to be a lot Bayonetta, but I haven't played that yet..

Memorable boss fights? Half of GoW III, jesus lol...

All of the Metal Gear Solid Snake Eater and Guns of the Patriots bosses were awesome. RE4's bosses were also great.

I'll also add that fire chick from Psi-0ps. The game glitched on me and the explosive tanks wouldn't rise out of the ground. I had to double lift them with tk to get them out and throw them at her, which took just a little too much time to not get killed.