Actually no airline is based on Tenerife Sur, but 15 other airlines are flying there from Europe. I am using the MD-80 family planes with which I had started, but I have now started to replace them with 737-800s. Probably looks wiser to find an other airport (e.g. Ibiza) which is closer to Europe as a second base.
Just had a look at Tenerife Norte. I mapped the demand to spain, which basically I looked at all potential destinations in Spain. With 737s you can link :
La Gomera(OK, just 80 demand, but for a 54NM flight, it's enough to make money)
El Hierro : you can easiyl set up 2 daily flights. Why just 5 times a week?
Melilla : 130 demand, really enough for a single aisle flight
San Sebastian : 100 demand, not that much, but still playable, without opposition
Pamplona : 240 demand, you can land 2 daily flights
Zaragoza : 220 demand; you can land 2 daily flights
Valladolid : 100 demand, you can land 1 daily flight
Santander : 240 demand, you can land 2 daily flights
Granada : 230 demand, you can land 2 daily flights
La Coruna: 270 demand, you can land 2 daily flights
Valladolid : 120 demand, you can land 1 daily flight
Asturias : 250 demand, you can land 2 daily flights
Santa Cruz de la Palma : 800 demand, you have 4 daily flights, you can add 3 more. Be sure there is always a 1-hour gap between each take off
Jerez : 250 demand, you can land 2 daily flights
Vigo : 250 demand, you can land 2 daily flights
Murcia : 130 demand, you can land 1 daily flight
Santiago : 240 demand, you can land 2 daily flights
Menorca : 230 demand, you can land 2 daily flights
Ibiza : 310 demand, you can land 2 daily flights
Valencia : 310 demand, you can land 1 more daily flight
Lanzarote : Why is your Thirs flight flying only twice a week? I could be daily.
Alicante : 300 demand, you can land 1 more daily flight
Palma : 210 demand, you can land 2 daily flights, at least one
Barcelona, 700 demand, You could have 5 daily flights instead of your current 2
Madrid, 1100 demand, can afford 8 daily flights, your opponent has 3, you have 1. You can land 4 more
Seville, Bilbao, Malaga at 70 demand are probably not a good idea.
Once you have filled all that, you'll think about a new base. 100/120% filling of demand is not a big problem, in this game. Even better : it deters opposition from attacking.