I was in Ubatuba for 5 days and lucky enough that I found about Gelateria Pistache through my…read morehostel (stay at Green Haven Hostel, amazing guys running it). I ended up going there four times because it was that damn good and tried several flavours. Don't be shy, the staff are friendly and will let you sample if you ask.
Pro tip - do one flavour at a time. So fresh and flavourful and cheap (6 reals for a single scoop).
I tried pineapple, pistachio (amazing), maracuja (passion fruit) yoghurt and a chocolate nut one. You can't go wrong! Definitely check it out if you are in town.