Obviously it depends on the summit.
Lightweight very old 3 element sotabeam - easy to assemble, compact, lightweight and has a handy fixing for the fishing pole.
Arrow dual band (2/70) antenna for satellite operations. Very lightweight and compact. A bit faffy in assembly but can be used handheld and in calm conditions can be taped to a mast using electrical tape or the like.
Tonna 9 Element Portable (!) beam - Not one for the multiband days or unless there’s more than 1 of you but in summer with an easy summit and the prospect of some E’s can get spectacular results. On one activation of G/SP-010 (Winter Hill) I managed 22 out of 72 qsos with continental european stations on 2m SSB (DL,F,PA, ON,SP)
I did try a simple cheap halo once, but I didnt have a great experience with it.
I’m surprised that more people don’t try vertical SSB as it seems an obvious solution to the antenna problem. I guess old conventions die hard.