I think there is no "amp to end all amps", even the Mark V is still deep in the Mesa territory. So I'm just gonna write down some tones that left an impression:
-Hearing a nearly-cranked Marshall 1959SLP through a full stack of Greenback-loaded cabs has left my perception of tone (and my ears) scarred for life. For simple, "just" rock n' roll, nothing compares. Unparalleled clarity, punch, roaring straight at ya at volumes that turn your brain into mush. Also, that's it's biggest (should I say loudest?) problem.
-The ENGL Powerball, a formidable metal amp - ENGL amps just have so much punch and smacking low end it's almost annoying at times, when you want a more saggy tone. Awesome clean in the same package. Couldn't coax a good crunch tone out of it though. Brutal thing, lots of gain (almost too much), maybe a tad on the compressed side, but that can be a good thing.
-Marshall JVM. Does take its time to dial it in, but it's fairly simple to tweak. With some creative EQing it can step way out of the Marshall territory for some very unique modern chugaloony tones. Aggressive as hell, like a seriously pissed off Rottweiler. Most awesome threshold of distortion/mild crunch and up to more filthy tones I've ever heard to date (at reasonable volumes too!). Also, it's got a nice clean that's hard to put a finger on. This would be my "take to a deserted island" amp.
-Marshall Vintage Modern. This is hard to beat for anything up to metal (maybe some lower-gain early stuff). EXTREME interaction between the guitar's knobs and the amp. Can get awesome and diverse tones out of it without even touching the amps front panel. It's like the JVMs Crunch tones taken out and redone to a whole new level. I spend ONE AND A HALF HOUR blasting every possible Zeppelin, Hendrix, Who, GNR, AC/DC riff I could think of and I could go on forever.
-TriAxis' Lead 1 RED (no Phat mod) through a 2:90 with Deep&Modern engaged, bass set high. King Kong ain't got nothin' on this bad boy. I'm not overdoing it here, our f*king bathtub was rattling at the other end of the house, and I was using a 2x12"!!!. Crazy low end, sizzling top, it finally showed me what all the fuss over the Rectos is about. But, like someone already said, needs volume to shine, otherwise it's kinda "meh, I've heard that in every GC tryout".
-Mark IIC+ voicing. Sounds so... simple, pure, straighforward. Beautiful midrange, which I think is a shame that everyone tends to scoop (at least using the Mid knob, the EQ is a different story). Gotta love it. Sounds great through a Marshall power amp too!
-Mark IV voicing too. Well-defined no matter what you do to it, a voicing where men and boys are separated, it "wants" you to screw up.
I'm just a 17 year old kid, so I don't have years of experience and tons of amps behind me, but I do like trying out gear (pain in the *** if you live in mfking Slovenia) and finding it's strong spots and weak spots, so I rarely label an amp/guitar/pedal etc. as "crap" or "useless". It must have at least ONE usable setting for SOMETHING, and if it happens to be what turns you on, it's all it has to be, the more of those settings it has, the better!
Plus, if you can get your tone out of (pretty much) any piece of gear (okay, you can't get Machine Head out of a Princeton, now, can you?), you're pretty much the winner! 8)