Nice/ Cote d'Azur, Barcelona/Costa Brava or Malaga/Costa del Sol. What's the best ending for a eurotrip in the summer?

I'm going to travel to Europe for the FRIST time this summer. Itinerary so far is:

Amsterdam 22/6-26/4

Belgium 26/6-28/6

Paris 28/6-2/7

London 3/7-7/7

But i'm don't know the last one. I'm planning to come back in 10/7, and the places i'd like to visit the most are Nice/ Cote d'Azur, Barcelona/Costa Brava or Malaga/Costa del Sol.

I'm traveling solo and want to have a summer/beachy experience but also to stay at a nice city.

So, whats the best ending?