What sounds best? A 1w, 5w, 20w, 50w, 100w or more? For me, it's a no brainer as the 100w always sounds best at low, med or high vol. 50s are fine but have something different going on; sweeter tone maybe but doesn't fill out the spectrum like a 100w. Regarding the lower wattage amps, sure they CAN do the job, most gigs BUT they won't sound as full IMO when you have a 100/50 at the same vol as the 20...the 20 strains and sounds much thinner than the 50, and even more so the 100.
100w amps always just sound best, at any vol, compared to any other wattage to my ears anyway. I buy my amps because they sound best for what I want to hear. I've also never had a hard time using any higher wattage amp, at a super low volume...they still sound great no matter how loud/quiet they are.
Sometimes people think that the only way a 100w amp sounds good, is to dime it....not even close to being a true statement. Every amp sounds its best loud, of course, but they can also sound killer down low. I had a Wizard MC100 that had this killer super low vol tone, whisper quiet that sounded like it was about to blow.