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
![tongue tongue](/images/graemlins/default/tongue.gif)
OTOH you could rely on the (allegedly) undestroyable (and nearly unpayable
![laugh laugh](/images/graemlins/default/laugh.gif)
) Rohloff gearbox - some of us do so.