Three very different places on your list. I've sailed all of them during a circumnavigation so, although not bareboat, I do have some knowledge of the areas.

Brazil... Don't know where the bareboating is but we were in Salvador and Recife, a couple of weeks in both places, and I wouldn't recommend them. Not great sailing and they were the only places around the world any of our group (we were doing the World ARC with around 25 other boats) suffered any crime. Two or three women had bags ripped from their shoulders and pulled to the ground in broad daylight and even though accompanied by men. We were warned in advance but it still happened despite caution and vigilance. It may be different in the Rio area but I'm not going back to find out!

Tahiti... Wonderful area with Moorea and other similarly beautiful islands not too far away but distances are much greater than say the BVI. Horrendously expensive food and wine (almost everything is flown in from France) but good quality. Bora Bora is an overnight sail away but I don't know if you are allowed to go that far with the charter companies. French Polynesia was our favourite area in all the circumnavigation. I planned to go back next year to charter a boat for three weeks and head from Tahiti to the Tuamotu Atolls but my wife's bad back now rules out sailing in remote areas so that is sadly off the agenda.

Tonga... If you really want to get back to basics then Tonga is for you. Wonderful, barely inhabited islands conveniently located not too far apart. Provisioning will be basic, restaurants are few and far between and in most places will be simple island food like Breadfruit baked in banana leaves in a sandpit. We were in the Vava'u area and Sunsail/Moorings were right there. September/October should be a good time to be there as it is dry and outside the cyclone season.

In summary, if you like a reasonable level of civilisation then Tahiti. If Anegada in the BVI is a bit too busy for you and you really want the south sea island experience, go to Tonga. If you choose Brazil don't say I didn't warn you!