As far as I'm aware, the worst months in the Canaries are Dec and Jan. We've been 8 times in Dec and once in Jan - all to the same area. Get to Tenerife, but make sure you go to the south of the island, as the north is notorious for being much cloudier. Anywhere around the Adeje area and you can't go far wrong. Temperatures are usually low to mid 20's with hardly any rain, but be aware that it's not without risk. One year when we arrived, we'd just missed the storms by a couple of days ..... the sandbags were still out on some of the streets, which had been placed to try and divert the flood waters.
From our personal experience, the weather has usually been like a nice pleasant summer day in the UK.