The problem with the "ultimate amp" for me, is that my favorite amps for every type of tone have different power sections and wattage. I have never played an amp that allows you to change wattage on each channel that actually could achieve this.
My favorite clean tone is the Deluxe Reverb. The 6v6 power section without a master volume that sounds epic when cranked is a huge part of this. You can put the preamp on a larger power section and it sounds cool clean, but you'll never get those pushed cleans correctly. This channel, I'd want to be able to add the bright cap in or out and I'd want a mid boost or a knob that can be turned on or off in the circuit.
My favorite crunch tones come from 50w British (
Marshall) amps with EL34s. 50w because I like a little bit of power sag so it thickens up and isn't too bright, but still enough headroom that it stays tight. I'm partial to the Plexi tone, but JCM is cool, too.
High gain, I'm split. I love hotrodded Marshall sounds, but I feel like this could just be switchable on channel 2, as I probably wouldn't change settings much. Maybe a bright switch and a mid scoop (contour) switch.
That said, historically, I haven't ever lived without a 5150 for long. I also dig boosted Rectos and German (
Engl/Diezel) voicing. These are generally 100w 6L6 amps, but I do usually prefer Rectos and Diezels with EL34s or KT77s. I've never played an amp that successfully blended a solid British and American/German tone in one - I've played a few that got close, but they weren't as good as just having one of each.
I also now find it weird when I play my amps that don't have a built in gate. I have a gate pedal, but the one in my Savage and Iconic seem to do something different? It's cool to have an amp that's silent at all times, too.
Everything would need to be MIDI switchable. Would also be cool to have a good built in load box, without the IR junk. My Engl Savage 60 Mk II is the closest I've gotten, but I still prefer my Bogner 101b or Armored Sabot for the Marshall side. It also can't quite get that 5150 sound, like my Iconic. If I had to pick one amp, I'd stay with the Savage, since it covers the most ground best. None of my channel switchers can even touch the Deluxe Reverb on the low gain, though.
I guess you could maybe do all of this with a weird power section like the Road King, but it'd be insanely expensive to retube and just seems like complete overkill. I had both versions of the RK and didn't feel like changing the tubes on each channel did much. There's obviously more to the power section for the different sounds than the tube itself.