I've stayed at a ton of different hostels around Bangkok over the years, but Suneta has quickly become my favorite! It's two sois away from Khao San, one of them being Rambuttri, but it's very quiet at night and I got some very pleasant sleep here.
I had a cabin room, and it does get a little warm in there so I kept the door open halfway at night which helped a bit. I think next time I will go for the curtain bed because of this, but it really wasn't much of an issue! The cabin beds have TV inside, and I'm not sure if it was just mine or all of them – but mine had no English channels, only Thai. But I didn't mind as I hadn't come there to watch TV. :) The air conditioning is on all day and the room itself is nice and cold. Be advised that in the cabin room (not sure about the other rooms), there aren't any lockers. This wasn't an issue for me as I just had a small backpack which I kept inside my cabin, but they do have luggage storage in the lobby.
One of my favorite parts is that have four absolutely adorable cats! They hang out in the lobby. Just watch that you don't leave any little things around, as they'll surely find it to be a toy! :) Free breakfast was simple (toast with butter and jam with juice) and they also have a paid option for 69THB which includes sausages and eggs. Bathrooms were NEVER crowded – toilets and showers were abundant and always clean.
Staff were so sweet. It was so nice talking with them in the morning and evening!