I have had pretty good experiences in all of them but I've only ever been to St Tropez once (too far away) and Beaulieu once (so tiny)
I love the Monte Carlo store - very friendly, usually a big choice and a nice large store. My 'local' store is Nice and I have had some lovely items from there, it's fairly small though and it's hard to browse in peace if you're not buying. A couple of young female SAs in there are particularly pleasant and friendly. Cannes, I don't get to much but if you do, ask for Thérance (?spelling), he is wonderful and speaks great English, nothing is too much trouble for him and he will get out every bag in the store for you to try/model etc without obligation. One of the nicest stores however is at Nice airport, I use terminal 2 so I can't speak for terminal 1 but the staff at T2 are great, particularly Marc and Pascale - Pascale knows a lot about scarves and is happy to share her knowledge. For a small airport concession, they have a surprisingly wide selection of scarves and small items but no Kellys or Birkins. They helped me to get the dancing horse keyring when I decided too late that I really wanted one and couldn't find one anywhere, also, little things like remembering your name and being pleased to see you and chat even without a purchase, I can't praise that little store enough, I've always felt welcome there.
Albanne mentioned Aix en Provence, I manged to call in there very briefly this summer for the first time, didn't buy anything but the SAs were very friendly and the store had a nice selection, I remember seeing a beautiful biscuit Kelly flat and a couple of other nice bags.
If you are hiring a car, you would be able to cover them all in a couple of days, allowing for the 2/3-hour lunches, lol.