Which app is best for buying concert tickets in Spain?

Ticketmaster is pretty much the standard in Spain for festivals like Primavera Sound, so you probably won't have a choice anyway - it'll sell them directly on their platform. That said, the experience with virtual queues depends more on the volume of people than on the app itself - when sales open for a big festival everyone deals with the same thing. What actually works is having your browser updated, being logged in beforehand, and having your phone or computer on a solid connection, because any lag can cost you the tickets.

Don't wait for tickets to come out on the official website because Primavera probably already has an exclusive deal with a specific platform - check the festival's page directly to see which one it is, since it's changed several times in previous years. What I do recommend is not to trust that one app is "faster" than another: virtual queues are a system, they don't depend so much on the platform but on how many people try to buy at the same time, so what matters is that you have your account verified days beforehand, your payment method ready and the app updated. Have you already checked on Primavera's website which one will be selling tickets this year?

For Primavera Sound specifically, you'll be stuck with whatever platform they choose, so the first thing is to go to their official website and see where they're selling tickets when they drop. What nobody mentions is that a lot of times Ticketmaster and other platforms have insane virtual queues but the real difference is in your connection and having your payment info saved beforehand; if you start filling in data at the last second you waste time. My advice: create your account ahead of time, verify your email, and have everything ready a week before sales open, no matter which platform it is.