Nat has been to gyms all over the world, so take his word when he recommends his top five gyms in Bangkok.
Health and fitness in Bangkok has come a long way in the past ten years. There was a time when belonging to a fitness club was only for local elites. With the opening of large fitness centres, membership has become accessible to practically everyone. The five below are my favourites:

Cascade Club
Take it from someone who has worked out in gyms all over the world. This is a candidate for the best fitness centre anywhere. The spacious weight area features state-of-the-art equipment and there are dedicated aerobics, spinning and Pilates rooms.
6&7/F The Ascott Sathorn Bangkok
187 South Sathorn Road
Bangkok 10120
Tel.: +66 2 676 6969
BTS Chong Nonsi

Studio by FIT
It can be easy to get lost in today’s huge fitness centres. If you want something more boutique, Studio by FIT is the centre for you. It is run by Fitness Innovations (Thailand), the place trainers go to get international certification from institutions like the American Council on Exercise (ACE), so you know they’re good.
884, 886 Mahatun Plaza
Phloenchit Road
Lumpini, Pathumwan
Bangkok 10330
Tel.: +66 2 650 9242
www.facebook.com/fitstudio.thailand?sk=info
BTS Phloenchit, Exit 2

source: http://www.facebook.com/nows.fitnessclub
Now’s Fitness
Looking for an inexpensive but comprehensive local gym? This is the place to go. The facilities are a bit smaller than what you’d get at a branch of the bigger chains, but the equipment is well-maintained and up-to-date. The location is convenient too: right at the foot of the BTS.
1/F Now’s Fitness Building
Phayathai Road
Bangkok
Tel.: +66 2 251 9557
BTS Phayathai, Exit 2

source: http://blog.travelpod.com/
Royal Bangkok Sports Club
This is the grande dame of Bangkok sporting clubs, with exclusivity to match. There is not only a golf course, but also a horse racing track. Throw in two swimming pools, grass tennis courts, badminton courts, a fitness centre, hairdresser and spa and there’s nothing like it in any city centre the world over.
1 Thanon Henri Dunant
Pathum Wan, Pathumwan
Bangkok 10330
Tel.: +662 255 1420 9

source: http://www.soidb.com/bangkok/sports-club/true-fitness-exchange-tower.html
True Fitness
This is the only chain fitness centre that I would recommend. Five floors of equipment and exercise space makes the Exchange Square branch one of the biggest in Bangkok. The equipment is well-maintained and the personal training top-notch.
LG-4/F Exchange Tower
388 Sukhumvit Road
Khlong Toei, Bangkok 10110
Tel.: +66 2 663 4999
www.truefitness.co.th/index.htm
BTS Asoke, MRT Sukhumvit
16/02/2012 - 12:23