That's hard to answer in such general terms. It depends a lot on what you consider "best". As cheap as possible? As accurate as possible? Some reasonable compromise?
As a rough guide:
"Strong flexible": cheapest, detail reasonable, but not fine, rough surface. painting needs special care
"Detail": medium price, good detail, reasonable surface
"Frosted Ultra Detail": more expensive, fine detail, good surface (let's hope recent technical problems have been solved)
For 1/72 scale vehicles, "Strong Flexible" should be good enough for most purposes.
For 1/350 to 1/700, even FUD may not be good enough for very fine detail, especially thin round parts, like handrails or the barrels of small calibre guns.
Combining various materials should be possible, but check the pricing policy. Each item price has a volume-dependent part and a fixed handling fee. If you combine different parts the handling fees may negate some of the savings from using different materials.
You will need some experience to know what to use. Check out the Shapeway shops (lots of vehicles and ships in different scales) and forums, there are quite a number with photographs and see what materials others use. Read the discussions about which materials others use and what tricks can be used to save money, e.g.hollow parts, sprues with multiple objects attached that count as 'single piece' for the handling fee etc.
Upload some of your designs, check what prices Shapeways calculates for different materials, print (or buy from other shops) some test models and decide what you like.
Greetings
Karl Heinz