I used to play Legacy although I doubt it's still around, if it is it's probably dead because what made it was the fact most of the top p2p clans from normal RS were on it, which kept up not only a good quality of player base but also a large one. Like decent 5v5s at MB.
Generally anything where you could spawn items and the gold got deducted from your inventory (in other words, the intention was non-stop PKing) I used to play. I think the more popular ones now are Ikov and Soul Split for the simple fact that they have a healthy enough population for you to PK literally all day.