I don't know if this is the brightest, but this is the one I just bought:
https://www.amazon.com/dp/B01HEF6R9Q...roduct_details. It's pretty bright, very hard to look directly at the light when it's on full brightness. It should be seen for some distance.
I didn't really have room on my seat post either, but the rubber mounting clip allows mounting on the seat stay, which is where I put it.