The Mark 1 eyeball is the best option of avoiding another aircraft.
The Mark 1 eyeball is the best
non-technological option of avoiding another aircraft. It is however pretty useless because an aircraft on a genuine collision trajectory will be a stationary point in your field of view (assuming straight line trajectories for both).
Gliders maneuver around so should be more visible, which is probably why there has not yet been a glider-GA midair, whereas GA-GA is about 1 per year.
The reason I haven't bought TCAS (£10k+) yet is because all but one UK midairs I know of happened below 1000ft (one at 1800ft), the vast majority of such low level traffic is nontransponding (such is totally obvious from flying under a radar service), and I never fly that low except when taking off or landing.