Here's a vote for horse. One small note on the poll - your last option, 'horsetail hair', is just a little bit inaccurate. Most horsehair brushes use a blend of mane and tail hair.
Can't really decide. If I'm shaving with creams, I use my Silvertip badger......if I'm shaving with soaps, I always use my boar. Haven't tried horse or synthetic but those are next on my list to try.....
I just started DE shaving and picked up a boar brush at the local store. Used that until the other day when my silvertip badger arrived. My first impression between the two were the badger was a softer feel on my face. The 2nd thing I noticed was I got a better lather with badger as well. I face lather so maybe the boar would be better in a bowl since the bristles are a bit stiffer. Anyway I prefer the badger but not totally against the boar.
Though I love my V-L White Horses and enjoy both Synthetics, Badgers have captured my fancy at the moment due to my choice of Best, Finest or Silver-Tip.
Voted boar because at the moment that's really all I use. I have a TBS Synthetic but I only use it if I'm travelling and don't think one of my boars will have time to dry.
Maybe my vote will change once I get a badger but the boars work so I'm not in a rush.
I prefer boar brushes. I've owned badgers, including silvertip, super, and best grades; but none do it for me like a Semogue boar brush. My current line-up includes 620, 1250, 1438, and 1800: each brush has a different feel, but all produce excellent lather.