When will you be there and how do you feel about heat?

While Andalusia is IMHO the most interesting part of Spain - it is hell in the summer with temps often over 100 and as much as 120. In fact, in summer, IMHO, the only really comfortable part if the NW Atlantic coast.

If you wish to immerse yourself into Castilian you should take those regions off your list which either officially or predominantly speak another language, i.e. Catalunya, Baleares, Euskadi/ Basque Country, Galicia, Valencia (to some degree).
Language-wise, Madrid should be an obvious choice.
Or M

oops.. guess I accidentely hit the submit button

Or Málaga, Salamanca, Toledo, Segovia...
The latter, "smaller" cities will make it a bit easier for you to leave town for outdoor activities, maybe.

If I had the choice I would follow my daughter's footsteps and head for Salamanca. Never in my many travels have I seen a city I liked as much, in Spain..size (small)!!!culture (university city)!!!, family oriented ( no teen age or otherwise groups hanging about) with much enjoyed parks for family holiday activities, good public transport for day trips.. no graffity on the antiquties, Language schools accredited, The list is nearly endless. Madrid is great but huge..South is too hot..Barcelona is too touristy, Costa del Sol too crowded and summer time HOT..San Sebastion, although awesome is basque speaking, Lots of choices but Salmanca wins my medal.

Thank you everyone for your responses! I would be there from October to May corresponding with the school year. Yes, I do wish to live in an area that speaks Spanish (Castilian) as that is what I have learned in school in the US and what would be most beneficial for me in the future. Thanks again for your responses. Hope to get some more input.

As already noted, if your purpose is to learn Castilian Spanish (Spain has at leat 5 major languages), then keep away from Catalunya, Euskadi/Basque country, Baleares, Galicia, Valencia and possibley even Asturias. Of all the places you might go, Madrid would be my first choice. It is the center of culture for all of Spain.

Unless your teacher spoke Castalian Spanish, what you learned at school is not what you'll hear in Spain. Sadly, most Spanish teacher are not from Spain, but South America, where the language is quite different.

Do not head for Andalusia either
https://www.youtube.com/watch?v=wJgr_mwcR1g

I studied in Granada, Andalucia and it was the best decision I ever made; friendly people, free tapas, the gorgeous Sierra Nevada mountains, La Alhambra, and an hour from the coast!!! Also a ton of students from all over the world populate the city!

Also if you can learn to speak Granadino Spanish, you will be fluent because it is basically Spanish but they don't finish their words like 'Como estas' sounds like 'Como esta' ... It's challenging at first but totally worth it ..and if your're serious about learning Spanish, not as many people speak English in Granada compared to a big city like Madrid.. especially if you're outdoorsy you'd love Granada ..it is full of parks and beautiful nature spots...

PS ..Andalucia uses Castillan ...I stayed with a family who spoke it there

I'd would recommend either Granada-I met many young Americans studying Spanish there, beautiful area, historic and nice restaurants and bars.

Or Salamanca-My college Spanish professor studied there and it was one of the best experiences of her life. Great college town.

Finally, I'm sure you realize that Spain Spanish and North American Spanish is different. The pronunciation of "z" and the use of vosotros(you plural) were the hardest to get used to.

I wouldn't be put of going to Catalunya or the Basque Country because of the second language thing. My experience was that when I was last in Barcelona, I heard much more Spanish being spoken in the streets there than Catalan. I found that the Catalans had no problems speaking Spanish to me - they did not insist on speaking Catalan or anything. They switched between languages without batting an eyelid. I would say that that Castillian continues to be a valid language througout Spain. It might be interesting to study in Catalunya etc as in addition to Spanish you could pick up a bit of Catalan as well. As far as the best region goes, I think it depends on the weather - plus some cities are cheaper than others. Whilst the coastal regions are nice (really nice in the case of the Basque Country), there is loads to do and see inland. It's a delightful country all in all!

As Jessica pointed out in the south of Spain, they often clip their words. Personally, I find it hard to understand.

Having spent time all over Spain except in the Valencia area, I actually would opt for Barcelona. And while Catalan is the dominant language, the Catalans take great pride in speaking an excellent and clear Castilian. And if your intent is linguistics you can also learn some Catalan, which is based in Provençale and live in the most dynamic city in Spain.

Along the same lines Santiago de Compostela would be interesting. There is an excellent university there and the climate is the opposite from the south. They speak Castilian and Gallego which a combiantion of Castilian and Portuguese.

Otherwise the heart of Spain would be an excellent choice.

Golemtoo They speak Castilian and Gallego which a combiantion of Castilian and Portuguese.
Interesting theory, but not correct. I will let WIKI explain.
http://en.wikipedia.org/wiki/Galician_language

LJaracz as you can see immersing in Spanish culture is not going to be easy. Spain is made up of a lot of different cultures. Every where in Spain you will find classes to teach you the language, everywhere you will be able to "outdoor activities". Maybe it would be a good idea to explain what these actives are...surfing, hiking...rock climbing, mountain biking...etc and could you also give us a clue as to how long you are planning to be in Spain and what time of year. I think we can then give you some definitive answers

Thank you everyone for you responses! I appreciate your valuable insights.

Thank you Ribe for the clearer unedrstanding.

From October to May there is no better place than Sevilla. That's where I learned Spanish and had the time of my life. Andalucian Spanish is similar to Caribbean Spanish which is what they speak here in New England (Puerto Rican, Dominican), very fast and clipped. I find Mexican and Central American Spanish to be much easier to understand. Spanish is Spanish, people from Barcelona and Peru understand each other perfectly well. The truth is, anywhere in Spain is a great place to live.

Egbert said it, anywhere in Spain is a great place to live, and everywhere they (also) speak Spanish/castellano and everywhere you'll find great language schools.

Andalucía would perhaps be the best option for the October/May part of the year. Perfectly fine chances for much sunny and quite warm weather also through December/March. Inland Sevilla and Granada are both wonderful cities, but for the "outdoor activities part" I would have chosen Granada with the Sierra Nevada and Las Alpujarras close by.

And I also have a special soft spot for Málaga by the sea, very laid-back and genuine Andalusian atmosphere. Here's a couple of articles about one of my absolute favourite cities in Spain after 25 years of intensive travelling in large parts of the country. http://www.telegraph.co.uk/journalists/nigel-tisdall/8773310/Malaga-Spain-Secret-Spanish-pleasures.html
http://www.guardian.co.uk/travel/2009/jun/21/malaga-spain-city-breaks-travel?INTCMP=SRCH

Great landscapes close by, such as the Serranía de Ronda mountain region, the quite spectacular Torcal nature reserve and wonderful Axarquía.
http://www.rusticblue.com/axarquia.htm
http://www.andalucia.com/antequera/torcal/home.htm
http://www.visitcostadelsol.com/municipalities/serrania-de-ronda

Thanks to everyone who has responded to this post. I really appreciate your time and insights. I'm still open to hearing more. Thank you!

After 5 trips to Spain,I find that my favorite venues all start with "S".
Sevilla
Salamanca
Segovia
San Sebastian

Good luck !

Bob..probably read my mind..Please add my name to the "S" list approvers. LJaracz you can't go wrong with these choices but my #1 would be Salamanca..Nice size, university town, young people galore and very pleasant weather..

A little joking, maybe not as much as it sounds:

If you plan to use your Spanish in North America where most Spanish I heard had one of the accents of South or Central America, you could opt for the unis of Las Palmas/ Gran Canaria or La Laguna/ Tenerife. IF those offer any of the programs you are looking for.

The Spanish pronunication of the Canaries is much closer to the South American than to mainland Spain - also due to centuries-long historic ties between the islands and South America (more Northern South America/ Venezuela, if I remember correctly). Even much of the vocabulary is S. American like banana instead of plátano or guagua instead of bus. So the Spanish you pick up there may sound more familiar or usual back in North America. (Or maybe that is of no importance)

Plus the climate is mild to warm and sunny all year long. No winter, no baking heat in summer, always hovering around 75F
Outdoor activities include anything from high mountain hiking (Tenerife's tallest peak has almost 4,000 meters) to windsurfing. Gran Canaria has one of the best spots for the latter worldwide.

Las Palmas and La Laguna are far away from the sterile holiday resorts on the Southern coasts of each island, respectively, so the living environment will be "normal" and not touristy.

But compared with Salamanca or any of the other cities mentioned above it would sure be a bit of an unusual choice.
Like learning English on Hawaii maybe

The downside sure is the islands' isolated location which will make you miss mainland Spain's sights and living in one of the glorious cities or which would often just be a day trip by train away. But from the Canaries it's a 1.5-2hr flight north.