Fastest, shortest or less fuel is the best in navigation for Garmin Nuvis?

And what about "less fuel"? Is it a combination between "fastest" and "shortest"?

I believe it's totally your call... "Fastest" will route you through highways and major roads where you'll reach point A to B by the shortest time possible. New nuvi's such as 37xx also take in to account of road speed limits to calculate the shortest time. The same route on "Shortest" may take a different route where the distance between point A and B is the shortest, still it could take longer time than "Fastest" due to junctions with traffic signals, slower road speed limits etc. "Less fuel" was made available specially for Hybrid cars. By using "Less fuel" nuvi will predict a route where you should use less fuel than Fastest. To use "Less fuel" you need to set up your car profile on ecoRoute.