Hangsen and the super concentrates that Steven Pye mentioned work great for me at 3%. (I think VapingZone calls them Super Concentrates, and EcigExpress calls them Flavor Express and Tobacco Express). If you're looking for the most concentrated flavors though, I think the Gourmet line at VapingZone would be the best. Many of them get great reviews, and people use them at 1% or less.