This is by far the easiest and fastest tool to use for cutting the top out of a keg. Use a 1/16 inch thick disk designed for cutting stainless steel.
I've tried the sawzall/jigsaw technique. Too noisy, too slow, takes too long and it's very hard to make a clean cut.
An Oxy-Acetylene cutting torch makes a total mess of stainless steel.
Plasma Cutter. If you've got one, go for it. I've heard that the splatter sticks to the inside of the keg. Then you still need an angle grinder to get rid of it. Hmmmmph!
Remember to let all the pressure out of the keg before you do anything!!! It's not worth getting killed.