The "best" depends on what the main light setup is. The relationship between hair light and main lights is a joint relationship; if you're using hard, directional lighting, like parabolic reflectors or beauty dishes, you can use a different type of hairlight than if the main light is a softbox. Harsh, direct, unfiltered flash as hairlight isn't a good mix with soft umbrella, softbox, or scrim mainlight sources. The following are for setups shot with soft-ish,diffused main and fill lighting,and are my own observations and opinions. I have used complete Photogenic and Speedotron lighting systems,and the following are my own experiences,and I have shot quite a few groups of 12 to 20, as well as thousands of one- to six-person groups.
1-One person. Speedotron 11.5 inch 50 degree reflector fitted with a 20 degree honeycomb grid, a snap-on mylar diffuser, and 2-way barn doors.
2-Group of 3 to 6. Speedotron 11.5 inch 50 degree reflector fitted with a 35 degree honeycomb grid, snap-on mylar diffuser, and 2 way barn doors.
3-Group of 12-20. Overhead 42x72 inch panel,white translucent fabric, two flash heads fired through panel suspended overhead. Or, alternately a bare flash head fitted with a Speedotron or Photogenic brand half open/half shrouded tube cover. One of the things about Photogenic and Speedotron higher-end strobe heads is that they have a BIG flashtube, unlike cheaper monolights that have a dinky, small,circular flashtube. With Photogenic or Speedotron pro-level flash heads, bare-tube flash is quite good as hairlight, and distributes quite well,and Speedotron and Photogenic both make special background/hairlight reflectors that shield 50 percent of the bulb, specifically for this type of use.
4-Snoots are a love/hate tool. They look good in catalogs, but I think most people actually prefer honeycomb grids, which are easier to control. I prefer a 7 or 11 inch reflector with barn doors and grids + diffuser material, or no diffuser, over snoots. I really don't think highly of snoots.