Den,
N85 without a doubt.
But, when you get to Gap you may as well run down the motorway for the last leg to Sisteron. Why? Both the superslab and the N85 run parallel along the valley floor, following the Durance river, so there is nothing to chose between them.
If you wanted to try something else (and have some time) go:
Grenoble
N85 to Vizille
Then pick up the D road - D529 (check the number I am looking at an old map) - to la Mure
OR
N75 to Monestier. Then - here's where it could be good and different -sideways on D34 via Mens, maybe to la Mure (goes backwards a bit) or Corps or maybe better still right down to St Disdier, then right on to Furmeyer. Hang a left on the D994, straight run into Gap and from there as above.
OK it misses out chunks of the Route Napoleon, but that will always be there for another day. The D roads are different, all green lined on the decent Michelin map. I've done them across that bit.