I know I'm inherently biased, but... go to Seville, man!

In all seriousness, it seemed like a lot of the cities I went to in Spain were all really similar, besides Barcelona and Seville. For what it's worth, I've been to Granada, Valladolid, Cordoba, and Malaga. There very well could be others that are completely different (including VLC or MAD) but that's my opinion of what I've seen. That's not to say that I didn't like those cities, they just didn't have much variety between them to me.