The room that sounds good for the purpose. Some people will record guitars in a hallway. Drums are typically best in a large room, though a small room with treatment on walls and ceiling, with a hard floor, can give great results. A small isolation box for a guitar /mic can achieve great results as well. It depends on what you are after, as well as what you have. Experimentation is the only way to find what works for you. I can tell you what worked for me, on a particular project, with my gear, in my room. It will probably not translate to your music, your gear, in your room. I had no idea what worked until I tried it myself. There is no 'right' room that works for everything. Just the room you choose for what you need.
I know it sounds vague, but it is like asking what color works for a good painting. There is no correct answer. You make selections based on what moves the end result. Yes, treatment of any room is a good start, but the tone you are after may just be a bathroom. None of us can tell you what you want. That's where your own knowledge, use, and creativity come in.