I have been at various times of year.
February most often (half term) the weather can be really nice, or a bit 'bracing'.
last year end of March. Absolutely gorgeous weather - and we were staying in the north side of Tenerife, which is renowned for being cooler than the south.
December - nice sunbathing weather- one or two rainy sessions.
September - very, very warm.
I have been to Tenerife about 6 times. My favourite resorts are the quieter places. Puerto de La Cruz is totally different scenery wise than the southern resorts- but may not be as lively for the DC. Los Christianos and Playa de Las Americas are the most popular- with a lot of activities for everyone.
Lanzarote - a few times there too. Costa Teguise is popular - quite a lot to do. Puerto del Carmen is the busiest resort but I haven't been so can't comment. Playa Blanca is a quietish family resort. My favourite.
Fuerteventura. I have stayed in Costa Caleta(used to be known as Caleta de Fuste) three times. Enough to do for families, but not 'buzzing'. Corralejo is livelier -but I have only been there for day trips. It's generally a quiter island overall- very nice though.
Gran Canaria - i only went once. We stayed in Puerto Rico - and tbh I didn't like it. Seemed to be a lot of touts, and it was just v noisy. But there are quite a few resorts to choose from- and i am sure that it's okay really.
The sea is quite cold even in summer. I do go in, but don't stay long. Some of the waves get quite high, and I don't feel very safe (chicken!). Hotel pools even if heated aren't always that warm - but I could be basing that on my February trips. :)