This is definitely one of the more critical aspects of choosing the best guitar amp, and it’s not so much based on opinion as it is your intent. If you’re just cutting your teeth practicing at home, that 30 Watt tube amp will be way too much. Likewise, if you want to gig or stay ahead of your drummer in the rehearsal room, that 20W solid-state practice amp probably isn’t going to cut it.
It isn’t just a straight line of more wattage equals louder either. Tube amp wattages are distinctly louder than their solid-state equivalents due to their analog construction. So a 15W tube amp will be able to compete with a 50W solid state in terms of pure volume.
There’s also another thing to take into account, which is headroom. Headroom is the amount of power a tube amp can provide before it starts to break up. Some amps have a lot of headroom, like the classic Fender Twin Reverb which you can turn up halfway without it breaking up. This means you can keep your tone clean and still be louder than your drummer. Smaller wattage amps will break up earlier, which may or may not be to your taste.