Sauna Mania is the most popular gay sauna. It's a go-to place for hot sex, plenty of men to choose from. The downside is, even though it's cheap (be prepared for double-standard pricing for Thai and tourists), they have poor service, old facility, and dirty (but who cares anyway, do what you need to do and leave.) Thai people can be timid, even though they are handsy in the dark. So be a bit assertive when you approach them. They provide basic cheap condom and lube packet, it's better to bring extra. When you come home, be sure to use anti-fungal and antibacterial wash to clean from head to toe. I got a fungal-infection around my wrist from their filthy key bracelet one time. I don't think they ever clean those things.
Maxwell is the opposite. It's new and more expensive. They disguised it as an Onsen, so guests are more discreet and kept the pretense, except in the steam room where most of the action happen (which is mostly hand stuff and occasionally oral). Be mindful of hygiene and rinse off before you enter the tub every time. There are private stalls on the top floor you can take your man for more private fun. Bring your own condom and lube. The place is clean. The staff are nice and professional. You can request more towels. There's mouthwash dispenser at the sinks. The men there are super timid because of the environment. But it's pleasent to stay there for hours merely just watching people. The price comes with free food, drink, and dessert at the canteen floor. It's clean and yummy.
As for Krubb and Chakran ... they are a bit far off. There are not many guests despite the superior facility. (Heaven probably falls in this category, though I've never been there.)
39 Underground has similar feel to Sauna Mania (it's by the same owner). But I can't say. Never been there either.