Much above a few hundred feet and small hills look flat - the landscape looses its interest. As for collision avoidance, best to avoid 1,200 to 2,500 which seems to be where most light aeroplanes fly. The RAF are usually sub 500, so somewhere between 500 and 900 works for me over open country. Higher for built up areas of course, or if the fields suitable for forced landing are few, or if I need the tailwind.
4,000 for aeros.
This is for a relatively slow VFR SEP. If I was flying a Concorde I wouldn't do that. Or the aeros ('cept maybe a roll or two!).