Due to the country's fairly pleasant year-round temperature, the best time of year to visit is largely dependent upon what you want to do and see during your holiday. Consider the weather of your anticipated regions carefully so that you're sure to pack everything you need to remain comfortable during your journey.
Travelers are often interested in visiting the mountainous Sierra, and with good reason—there are quaint, cool cities up there and some seriously spectacular landscapes. Most of the Sierra strikes a nice balance between warm and cold. It’s warm enough to not be freezing, but still cool enough that you won’t break out in a sweat each time you step outside. In fact, this region is known for its pleasant, spring-like weather. The temperatures can vary greatly, however. It usually warms up quickly in the morning—daytime temps hover around 70˚F (21˚C) throughout much the year. As soon as the sun goes down, it gets much cooler, and nighttime temps average 46˚F (8˚C). The dry season in the Sierra lasts from June–September, while the rainy season runs from October–May. Rains usually fall in the afternoon during the wet season.
Ecuador’s coastline sees more extreme wet and dry seasons, which is largely the result of weather patterns being influenced by ocean currents. The coolest and driest time of year is from May–December. January–April is usually hot and rainy. During this time of year, there are daily showers, with daytime temps averaging around 88˚F (31˚C). What’s more, every few years El Niño comes along and adds additional downpours in April and May.
The Amazon is hot and wet for most of the year. The heaviest rainfall occurs from June–August. Rains usually come during the afternoon, with massive downpours that last for an hour or two. After the storm passes, the evenings tend to be cooler and fairly dry. The driest months in the Amazon are September–December. Daytime highs average 88˚F (31˚C), while nighttime temps usually drop down to 68˚F (20˚C).
Once you've established when to visit Ecuador, be aware that whichever region of the country you choose to go to, you should not have to worry about the weather prohibiting your preferred method of transportation from arriving into the country. Speaking of, official entry requirements involve little more than a valid passport, proof of onward or return travel, and sufficient funds, so it shouldn't take very long for you to begin exploring.