I use Hertz SXM but everybody has their favorite go to rental company and I'm sure you'll get plenty of suggestions here. You'll just have to decide which one most suits your needs and budget.
As Carol said book your rental car right away. We recommend Kenny's Car Rental. We have used them the past few trips and couldn't be happier with their rates and service. All we have to do now is send Lesley Bruce an email with our dates and flight numbers and then meet him at the airport parking lot. The paperwork takes minutes and we are on our way. At the end of our trip we just drop the car off in the same airport parking lot and walk in the airport terminal. Car rental made simple and easy. No shuttles.
We all have "our guy" !! For us, the service you can't beat is from Sax rental company!!! Wide selection of new cars and lovely people to do business with!! Ask for Cindy, she and her husband Anil will take very good care of you! https://www.saxcarrental.com/
Feel like "newbie," as it's been way too long since in SXM! Hopefully all goes well with new knee for my 7th visit! Probably less walking ... but know we'll be very comfie at BSV!