...that's a tough one!
Everybody has their own favorite.
From the easy accessible spots, with no issues of sort (trespassing, safety, being kicked out), my preference would go to Christopher PG (Basic Training was just there today), and probably the Lower Presidio WWII battery area (Girard Road).
Take the k or m line to where they split and walk down to ocean ave at juniper Serra
Never had a problem training just myself or with one other buddy.
Three traceurs always equals handcuffs there though. Some people consider this place to be worth t though