Hello paulw28627. and welcome to the forum,
St Remy is my favorite town in the Provence area. It is charming, small but large enough for multiple restaurants and several nearby attractions - Glanum, van Gogh asylum, Les Baux is a few minutes away by car. Market day in St Remy is a great experience. Be sure to figure out parking when you book a place in St Remy as all in town lots are pay and they will tow you if park where the market is on market day. Huge free lot on the edge of town.
Avignon is the biggest central town and is a good base if you don't want to have a car. It is short on sights but has many places to eat, shopping and a good Les Halles permanent marketplace.
I really like Arles but it is more blue collar and less polished than the more touristy towns. It has strong Van Gogh connections and Roman ruins.
Both Arles and St Remy are about an hour or so from Aix. I haven't been to Aix but it is - by reputation - a more upscale and chic city. It is much closer to the famous coast locations like Toulon but further from Nimes the other Provencial towns like Avignon.
I would choose St Remy, followed by Arles and the either Aix or Avignon depending on your interests. On our trip earlier this year we stayed a few days in all three as well as L'Isle-sur-la-Sorgue to explore the area and some of the Luberon.
Hopefully that helps, have a great trip,
=Tod