In Spain San Sebastian has the most concentration of Michelin star restaurants. Most mentioned by a poster above.
However, there are great restaurants all over Spain. Like the exceptional Goizeko Wellington in Madrid or Oriza in Sevilla to name a couple.
BTW, this morning the NYT's reported El Bulli will be closed the next 2 years. Adria wants to experiements with new molecular food.
Sorry, but this just seems a bit lazy - have you searched this board? What do you mean by best? Are you talking about "money no object" best or "world renowned gastronomic" places? Or the best overall experience? Or the best value meal, where the food was still amazing? Or the best unheard of local restaurant? Etc etc.
Well you could simply go to the seven Michelin 3 Stars: Akelare and Arzak in San Sebastián, Martín Berasategui in Lasarte-Oria, El Bulli in Roses, Can Fabes in Sant Celoni, Sant Pau in Sant Pol de Mar and El Celler de Can Roca in Girona.
There are then eleven 2 Stars so that should give a top ten. But as Theresa said more info and a bit of research would help.