If you end up traveling to Thailand at all there is a better than average chance that you are going to end up in Phuket at one point. For many, they end up really enjoying this place and keep returning year after year. For others, they find it to be a bit much of a tourist trap and I can certainly identify with this notion as well. It certainly can be annoying to not be able to relax anywhere without being bothered to buy something every time you go anywhere.
If you can manage to see past all that though I really believe that Phuket contains some of the best beaches of anywhere in the world.
One of my favorite beaches on this island is Nai Harn beach far in the south of Phuket. This beach, unless you are told about it by someone else, is something that you are unlikely to be offered by an agent or even most of the people that ever travel to Thailand. For many it is an unknown place because there aren't massive roads leading there. This is precisely why I like it so much.
Phuket generally speaking is overcrowded and there isn't much that can be done about this. They have already put as many roads on this island as it can take and the time stuck in traffic is just something you are going to have to accept about being there.
If you can endure the trek down to Nai Harn which will take you almost all the way through the island, you will get a great surprise at the end.
There are only a couple of big hotels here and most of the accommodation is smaller air BNB type establishments that I think a lot of people prefer. If you want to stay in a giant hotel with multiple swimming pools you are unlikely to find that here but on the flip side the beach is astounding here. Be warned though, despite the best efforts to showcase the beach here as calm, it can be a bit dangerous and is home to some pretty gigantic waves during certain times of year. I enjoy surfing so this works out wonderfully for me.
Even if you end up staying on one of the other more populated beaches here, I think it is worth getting a taxi for the day to see what you are missing out on. Restaurants are plentiful here but it isn't as much of a hassle as Patong or even Kata beach is. For the most part you can wander the side streets without being pestered at all by shop owners whereas in other beaches his constant touting begins immediately as soon as you leave your hotel.
Normally when I am visiting I am in my friend's car so we don't have to deal with this but for the average tourist it can be extremely annoying and they won't stop, it is a game of numbers for them after all and they need to make some money too.
In Nai Harn this doesn't really happen because of the lower numbers of people there and the fact that the people who do work there tend to live there locally. Therefore there isn't much inclination for anyone to wander on over to this distant beach in the hopes of selling a few sets of sunglasses or flip flops. Those people stay in the much more crowded beach areas as well.
If you find yourself in Phuket, which I recommend you make happen generally, please do yourself a favor and head down to Nai Harn. In all the beaches that I have been to in my life this is without question in my top 3.