Are you talking about in the context of friendlies or tournament matches?
The term "legal stage" is up to interpretation, since each region tends to have different stagelists for PM; the metagame for stages has not yet stabilized such that there's a unified stage list, so it's up to the TO to decide which stages to use and which stages to ban. If you're talking about the stages on the front page, even some of those stages are questionable - Metal Cavern, for example, gives characters very little room to work with, whereas Rumble Falls, in contrast, has so much room for maneuvering such that camping is a relatively powerful option there. As such, there are many personal and regional preferences for which stages are used, but people tend to have the least qualms with Smashville and Battlefield because both provide an appropriate amount of space for defensive and offensive play, remain relatively static in terms of layout changes, have platforms to work with, and have blastzones of moderate size. (From my experience, FD is generally frowned upon because of how the layout makes chaingrabs a very powerful option.)