Ecuador can be visited all year round. Due to the equator running through the country the temperatures do not fluctuate that much throughout the year. However, there are large differences in temperature throughout the country due to the different altitudes.
The highlands which includes Quito, Otavalo, Cuenca and the Avenue of Volcanoes have warm days and cold nights. January until May is slightly warmer, but wetter, than the second half of the year. This is, however, the best time to visit the Galapagos, so if your trip includes the Galapagos, many people will travel to Ecuador during this period.
June to September are the driest months on the mainland and you will see the most sunshine during these months too. This is a great time for outdoor activities and bird watching. This period is also the best time to visit Peru, so a combination tour of Ecuador and Peru works well in these months.
The Amazon remains hot and humid all year round. Although the Amazon generally receives a lot of rain no matter what time of year, the wettest months are from April to September.
If you are visiting the Ecuadorian coast then the best time is during the warmer season from December until May.
October until December is also a popular time to visit, after the period of choppy seas that the Galapagos experiences from July until September.