Sevilla
Granada
Cordoba
Mallorca
Madrid
Barcelona
Valencica

I give the thumbs up to Valencia too. I thought it was a beautiful city with lots to see and do.

I haven't been in Sevilla, Granada, Cordoba so can't comment on those.
My favourites: Madrid, Segovia, Salamanca, Caceres, Los Cristianos/Tenerife
OK: Barcelona (nice but somehow a bit stressful)
Least favourite: Valencia, Cuenca, Bilbao

Hey somebody mentioned Cacéres already, and he is completely right (if you choose to go there, which is a bit of the beaten track, then also take a look at Truijlo and Merida.)
I think Madrid, Barcelona and Granada are the must see places. Toledo, Segovia, Aranjuez, Chinchon, can all be done as daytrips from Madrid.
Bottomline is there is enough too discover in Spain, to keep you happy for at least a month (and then Portugal is near as well!!!)
For transport you will find that flying can also be a good way to get around in Spain. there are a number of budget airlines (I think Vueling is the cheapest in general), but also Iberia gives good prices for their destinations within Spain. (Check: www.flightassistent.com</a> for your options from any given airport in Spain.
For me the top 5 would be:
1. Madrid
2. Granada
3. Toledo
4. Barcelona (it's good, but not so Spanish)
5. Caceres or Salamanca