The best time to visit Ecuador mainly depends on the region you are visiting. The different regions of the country have different climatic conditions. There are about four different climatic types prevalent in this region and the altitude plays a major role.
If you are visiting Quito then avoid the rainy season and opt for the dry season. The dry season is during winter that lasts from June to September. The temperature is quite low on high altitudes but is bearable in all other areas.