Either a dipole or an end fed. This is what 99.9% of all SOTA activators use.
I most often use a linked dipole for 20 / 30 meters. Cost a couple dollars to make and works great. You can add a section for 40 but I find it hard to get that much wire in the air on most summits. Nice tool for building linked dipoles here:
If you construct the dipole well, it requires no tuner on the band(s) it is constructed for.
The other antenna I use frequently is an end fed with 9:1 UnUn. This antenna requires a tuner but is easier to put up on forested summits and doesn’t require messing with links to change bands. Also costs very little to construct, but is a little more complicated than a dipole.
Search on the reflector and you will find thousands of threads on this very subject =)
I suggest you build lots of different antennas and try them all to see which one you like. Here is another similar thread with a lot of good info:
Good luck!
-Josh