It's not the amputation scene that is the best bit of the film, but then run up to it.
Ralston/Franco attempted to cut his arm off 2 days in before realising his knife was too blunt from chipping away at the boulder. He then tried again and realised however hard he tried he would not get through the bone. The best bit of the film is when he managed to suss it out. He has to break the bones in his wrist so he can cut through. The smile on Francos face is unforgettable, and one that Ralston had a big part in. Ralston said he smiled when cutting into his arm because he finally knew he was going to get out. He made sure Franco smiled during that scene. It was also done in one take, impressive to say the least.