Probably a JCM 800. Not because it's not a great amp, but because it's just not my thing. But the way many people make it seem like it's the ultimate amp and the father of all high-gain... er...
The thing is, for me, I've always been more used to dialing in amps like Rectos first, and then 5150's, and the EQ knobs and whole voicing of the 800 feels super alien coming from those. Low end is thuddy rather than deep or punchy, and the bass knob doesn't seem like it's very powerful at all. Midrange knob feels like, if boosted, makes the amp feel more stuffy, but if cut, feels like it cuts all the midrange aggression of the amp. Treble and Presence on one can really rip your head off, and once you come to a happy balance at one volume level, raise or lower the master just a little, and the whole EQ'ing of the amp changes drastically. Now, all tube amps do this to varying degrees, but I feel it's particularly bad in an 800.
JMO.