My ranking considers "best" as "I got what I wanted out of it straight away",
and "worst" as in either bad quality, or simply a too picky unit that didn't work for me.
Best:
Bogner Shiva combo. The best tone I've ever had.
Egnater Tweaker 15 and the 88 heads. The closest thing to the best tone I've ever had. Smooth and warm sounding units. I read somewhere that the Tweaker circuitry is based on a Shiva and I kinda believe it is.
Orange Dual Dark 50. Freaking brutal. Dense and thick and rich sounding, tons of gain and flexible enough.
Jet City JCA50H. The way I like my "tight" heavy sounds to be like. A nice cross of vintage and modern heavy sounds.
Peavey 6505+. Crazy loud head that needs no overdrive pedals or anything, just a great guitar and it slays right off.
Worst:
Laney Ironheart IRT60H. Atrocious cleans and fizzy gain, it sounds dead to me.
Blackheart Little Giant. Never got the appeal.
Anything from the Tubemeister/Grandmeister range fom Hughes & Kettner. Too modern, fizzy and overly tight. Very clinical cleans. Excellent amps, tailored for me NOT to like them.
Marshall TSL100 head. Nothing too appealing or terrible, but being a flagship unit I expected it to kill. It didn't.
Mesa-Boogie Triple Rectifier. Can you say learning curve? Too much for me.