A lot depends on what you're hoping to do on your island visit. If you want to lie on a beach, Fuerteventura has by far the best beaches. If you want to drive around and look at scenery, Tenerife or Gran Canaria would be the choices. If you want night life, Gran Canaria or Tenerife again. In my experience, Lanzarote isn't particularly attractive.
If you're decided on Tenerife, hire a car and drive around the island. There are some great sights. Be aware that a lot of the tourist resorts are fairly dreadful places. There's a lot of traffic in the tourist areas, parking is difficult. The beaches on Tenerife aren't great.