Max is 15W, and only from "MagSafe-certified" chargers (Apple, Belkin, Nomad, Anker Cube). I personally use the Belkin 2-in-1 and 3-in-1's, which are pricy but a lot cheaper on eBay. Only the Anker Cube charges at 15W, the rest will charge at 7.5W.
Any other "15W magnetic charger" will only charge at 7.5W because iPhone requires MagSafe certification to run at the full 15W.