Lot's of good input by lots of people who know what they are talking about. And as always, for portable operations (my emphasis), it seems to boil down to efficiency vs. ease of use.
Both Steve Weber KD1JV and Ron Polytika WB3AAL, who have done a lot of operating from the Appalachian trail, point out that while classic dipoles may be the most efficient antenna to use, there are practical logistical problems associated with them. There may not always be trees available, and even when there are trees available, there may be so many and so close together, that hoisting up a dipole may not be practical. I have to agree with Steve that hoisting a dipole or doublet in the classic sense, in a portable situation (especially when you are by yourself) can be an experiment in frustration.
Steve is a proponent of the End Fed Half Wave, while Ron likes a version of the portable vertical that he has designed and yields quite acceptable results for him. I have used both and personally prefer the end fed wire for the ease of deployment. Don't get me wrong. I have verticals antennas and love them. My Butternut at home and my Buddistick on top of the Jeep have both done very well for me. But as always, the ground plane is crucial. Close to 60 radials at home and the Jeep's metal body acting as a ground plane for the Buddistick make all the difference in the world.
The thing that surprises me though, is that when speaking of dipoles or doublets, everyone always seems to think of them in the classic flat top or Inverted Vee configuration, which of course, requires three supports. I have used doublets, such as the NorCal doublet as a sloper and as a vertical dipole with a modicum of success. My first Flight of the Bumblebees effort used the NorCal Doublet as a sloper and I was quite pleased with the results.
What it boils down to, of course, is that you have to try different things and see what works best for you. There is no single correct answer to the question of "What is the best portable antenna?" What will work in one situation may be totally unsuitable for another. No archer carries only one arrow in his quiver. Hams who are adroit in portable operations always seem to be carry more than one antenna configuration into the field with them, as long as they meet the requirements for portability and ease of use. And I think all Hams who love portable operations are on a constant quest for the "Holy Grail", an antenna that is lightweight, quick and easy to deploy, and will work as many bands as possible.
72 de Larry W2LJ
QRP - When you care to send the very least!