By far the best Mexican restaurant I know, not only in Barcelona! I never skip an opportunity to visit this place when I'm around. Nino Viejo is a spinoff restaurant of the famous chef Albert Adrià. The open kitchen is run by 8 chefs. So if you're seated right, you can look into the busy kitchen and learn a thing or two about coordination and teamwork. I always take the tasting menu. This way, you don't need to make decisions and you get to experience a lot of different dishes.
The price is reasonable. For about 130 euros for two, you can go full option, meaning cocktails, wine, tasting menu, dessert.
Make sure you make a reservation 3 weeks ahead!
If you're able to spend more money and want to have a crazy Mexican experience, make a reservation in Hoja Santa. This is the haute-cuisine brother of Nino Viejo)