Are you talking about putting a reverb directly on the mixbus? Or are you talking about a global reverb that you are sending multiple tracks to? If you're talking about the first one, IDK.. I've never put a reverb on the master track.. but as far as global reverbs, it's really dependent on your taste and the song. As far as free, just do a google search and start downloading them and trying them yourself, that's going to be the best way to decide what you think is best for ANY plugin. There is no "best." You'll hear time and time again about pros using certain cheap stuff because they like how it sounds.. it's just about developing your own taste and what YOU like. It's rarely about what you use, but how you use it.