The simpler, the better.
E.g. I would recommend primitive thumbshifters, no STI levers.
The rest depends on your purposes, but I think that a standard three-ring-eight-cogs setup will be the easiest to be repared in any corner of the planet
OTOH you could rely on the (allegedly) undestroyable (and nearly unpayable ) Rohloff gearbox - some of us do so.