Wire is graded by the number of strands and the thickness of those strands. Home Depot stranded wire, for example, has a smaller number of thick strands which is suitable for low vibration applications, like houses. Where you incur maximum vibration, like a generator and main engine starters, you likely want as flexible a wire type as possible. If no one has a specific recommendation here it might be a good idea to get some professional feedback say from a genny installer about what wire he prefers.
I know welding wire is very thin stranded and very soft and flexible and others have mentioned they have used it for battery cables, but I'm not positive enough about it to recommend it here.
One thing I did on my genny when I first bought my 1972 Hatt was to fold some (clean) absorbant oil pads under a place where friction was wearing on the main genny wire insulation. I tied the pad in place with some plastic cable ties and that soft "bed" remained so for the 11 years I ran the boat. You'll also want to be sure the rubber inserts in entry/exit holes at the genny are still in good shape..otherwise those metal edges, if exposed, will cut thru the wire.
