I have had pretty good luck with Adventure Lights Guardian LED.

They are small, light and relatively inexpensive. The only draw back is they use coin style batteries, although a spare pair of batteries takes up very little space.

I have used these to both mark people and control points with good success. For their size, they are extremely bright. I haven't actually conducted a controlled test with these, but the green and white strobes have both been bright enough to watch students walking along a trail over a mile away.

Another more robust option that I have recently been using is the S&S Precision V-Lite. It isn't as versatile as the Guardian, but I like it better for attaching to a helmet or pack. Below is a picture of V-Lite on my helmet at the Monument Fire earlier this year.