I have been asked over the years, “When is the best time to visit Rio?” I always jokingly respond, “Right now!” There are however, best times to visit, and I base them more on the weather and in-flux of people than anything else. You really don’t want to go during the hottest months of the year (Jan-Feb) unless you are going for Carnaval. You as well do not want to go when it is super crowded unless that is your thing. I have lived there 10 years and can tell you that these periods are the most uncomfortable (hard to get a spot on the beach or a table in a good restaurant) I think that October-November, (especially the latter) or March-April (in this case, the former) are the best months to go. The weather is nice, but not too hot, and the natives are in a good mood, but not stressed. If you are going during Carnaval or for New Years, take a suitcase of patience with you, go to restaurants a little earlier (9pm) and avoid the busiest hours on the beach.