I suggest Salamanca, but the only drawback is that there are, at times, too many non-Spanish students there.
The university's prestige, and the opportunity to hear the local castellano are indispensible, and should override the fact that every other street corner would have some borracho from the US puking his parents' tuition money into the gutter.
I love Salamanca, and the city is, in and of itself, a treasure.
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Ongi etorri!