What is the greatest boss battle of all time?

A pretty simple question, many legitimate answers. Personally, The End from Metal Gear Solid 3: Snake Eater wins hands down. After two and a half hours of slowly grinding him down I was met with an incredible sense of accomplishment followed by my taking a little break because my sweaty hands were aching after gripping the controller for too long. I bought a(nother) PS2 for that game, and even though back then $300 was a lot of money, it was totally worth it. In fact, MGS3 probably had the best cast of boss battle in the history of video games. What do you guys think?