I am far from an expert on what specific beaches are better than others - and I do think it is a matter of opinion and what particular day you go - but I spent a lot of time at Camilo and had a great time. They can all get pretty busy on nice days but still remain very nice and relaxing, and you can swim from beach to beach very easily.
I absolutely love Lagos, it's one of my favourite places in the world. If you haven't booked your accommodations yet, I highly recommend Casa dos Cedros. It's a really cute BnB with a pool in a nice neighbourhood. It's a 15 minute walk from Porto de Mos Beach which is a really nice, quiet beach with two beach side restaurants. My favourite beach in Lagos. We also loved a restaurant called Casa do Prego which has amazing food and a great vibe. You'll have to book a reservation in the restaurant a couple days in advance because it gets busy.
We also booked an absolutely insane hike on the coast of Portugal through West Coast Adventure Co. We've been to Portugal twice and the hike was the highlight of both of our trips.
You'll have an amazing time! Let me know if you have any questions.