Hmm, not the first to say this - 'BEST', let's say great for 'Rock' and then throw in a 'In My Opinion' in front of it..
JCM 800 for me. And if you're splitting hairs, the 1983 model with the 6 filter caps.. The later ones with 3 filter capts can (IMO) start to get grainier and thinner sounding the more you turn them. (There is also a 5 filter cap model between '84 and '86 I think) But they still sound really great. With these, amps (just like most of the earlier marshalls), you need to run them loudish to get a fantastic tone.
The '77 and '78 Master Volumes are also great amps for Rock.