T5B > T5GCS > T5GCN, with the caveats above about visiting T5B if you are unfamiliar with T5.

I would therefore probably recommend T5 galleries south (as you are likely to leave from T5A gates) which is on the top floor of the main lounge complex and, despite anything you might hear on here, compared to the majority of business or paid lounges around the world is excellent with plenty of space, great views of the runway from the south part and a good selection of food, drink and alcohol.
Galleries club north on the terrace has even better views (probably the best of any lounge at T5), but does tend to be busier.

If you are happy to follow the directions about using the tunnel walkway then by all means go for T5B.