Along with its competitor, Yasuni, Cuyabeno is the best place in Ecuador to see animals in their natural environment rather than in a zoo. However, the tropical rainforest comes at a price. First you have to get to the reserve, in the east of the country, find somewhere to stay and employ a guide. You also need to hire a boat to go on trips on the river.
I had the opportunity to go on a similar trip to the Amazon in Bolivia for next to nothing, where in three days I saw as many animals as at Cuyabeno: monkeys, crocodiles, caimans, birds of all sorts and even pink river dolphins. If you have the money, Cuyabeno is a fabulous experience, especially for families.
Even though Yasuni is bigger, many tourists just as impressed by Cuyabeno, which is said to be more authentic.