Signature drink in Spain?

"Vermut" is a strong, sweet wine usually served over ice, sometimes diluted with water (if it's too strong for you), often drunk as an aperitif, so that could be it. Cava is the Spanish version of champagne. You can also get sangria made with cava rather than red wine.

Was it Rioja?

Up in the Basque region the pintxos are often accompanied by the local, slightly fizzy white txacoli.

It's hot and the drink we consumed prodigiously was tinto verano (sp?). It's red wine with lemon soda, Fanta or similar. Spaniards favor it over sangria it appears. I know, wine with soda, not cool. Try it, you will see, it's terrific.

Dark drink? Hmmm, chocolate con churros? Hardly a drink though. You can find it in many places in Spain, but the best I had was at a little family operation off Placa de la Alfalfa in Sevilla.

.... Spain is NOT a country in the traditional sense, be regarding food, regarding drinks or regarding anything else. While the country is politically divided into 17 autonomous regions, in practice there are four historical nations: Castilians, Basques, Galicians, and Catalans. Each one has its own "ways": its own roots, history, traditions, cultural references, areas of influence, etc.... even different languages are spoken! What I am trying to convey is that there is no single anything that can be defined as "Spanish", there's very little or no uniformity, so that'll depend very much on the area you are talking about. Obviously, though, there are some coincidences and similarities but that's about it, very often it's like talking about different countries. Any expat that has lived long enough in two or more of these territories can tell you that.

Having said that, for many, using the term "Spanish" is equivalent to referring to traits, traditions and/or typical stuff of those regions historically dominated by Castillian culture (Castille, Extremadura, Andalucía...) therefore one could say that "Spanish" is equivalent to say "Castillian", but of course, that wouldn't necessarily mean it's the same in the other historical nations. But again, this is a very sensitive and highly controversial issue over here so best not to delve into it here and now.

In Basque country the 'signature drink' may well be the txacolí wine, in Catalonia, I'd be inclined to vote for either the vermouth or the cava, in Andalucía probably the fino... And then there are "drinks" -not necessarily 'signature drinks'- that are common in one or more regions, but not general enough to label them "Spanish" either (ie. sangría, tinto de verano, kalimotxo, sidra, orxata, rebujito, etc.). These tend to be mixes of non-alcoholic drinks and alcohol (ie. rebujito is typical in Andalucía and it's a mix of manzanilla, a sort of sherry, and a soda drink, normally Sprite or similar).

Confusing?... I'm sorry for those expecting a more straightforward answer... "Spain" is not a straightforward country either! :))

I was introduced to the Kalimotxo while on the Basque tour last month. Cheap red wine and Coke. Very refreshing.

A quick summary is right here:
http://drinks.seriouseats.com/2014/04/wine-from-spain-label-regions-rioja-beginners-guide.html

In line with what Enric, Chani, and others said above, there in Barcelona you might aim for a house version of a vermut and when you get to Madrid you might aim for a tinto de verano. Both are refreshing mixed drinks that aren't too strong. Spritzers with style, you might say.

Typical house wine in Madrid is Rioja; many spots will also have slightly pricier crianza and then a reserva and gran reserva choice in Riojas if you want to spring for something with a little age to it.

To me the current pleasant surprise is the quality of the rosado wines available--I was very impressed with the house choices all around Catalunya and Valencia this spring; here's a little slightly dated article on the topic:
http://planetavino.net/2013/09/02/misunderstood-spanish-wines-rosados-and-roses/

Hi @avirosemail... funny thing though is that "rosats" (in Catalan/Valencian, or "rosados" in Spanish) were not much considered up until recently, in fact, for many are still not... wines are supposed to be either blanc (white) or negre (literally "black" albeit you guys call them "red"). But I agree with you, there are excellent rosats out there.

As per vermouth, this a good list of places: http://www.timeout.com/barcelona/food-and-drink/vermouth-the-happiest-hour-en_gb And talking about "vermouth", this drink has made a revival these past years. Not the first one I must say. The drink has been traditional in Catalonia for decades, with many vermouths house-made in many small bars and manufacturers across the whole of Catalonia. An area with a great tradition and many household names is around the city of Reus, in the south of Catalonia. Vermouth was en vogue during the 1910s-20s, then again in the 50s-60s... In Catalan culture, an expression was adopted "anar a fer el vermut" (literally: 'to go for a vermouth'). Funny enough, the translation of that expression has evolved into more generic 'to go for an appetizer'... and normally (albeit not always) just before lunch. So these days, when Catalans say "anem a fer el vermut" (let's go for a vermouth) this should be understood as "let's go for an appetizer -and not necessarily a vermouth!-, an a tapa or two, then let's go to lunch". Also, "anar a fer el vermut" is rather a week-end activity since one doesn't have to go to work after.

The vermut I had was very dark in color.

Vermut can be dark or light. Slightly different effect. Traditionally served on the rocks so it can be very refreshing.

Moving the conversation north across the border into France, there if you want a sweet vermouth for an aperitif, you ask for a martini. The Martini & Rossi brand has become the icon for sweet vermouth, even if you are drinking a different brand.