Regarding most popular I'd have to say Fastrack because that is what any new hobbyist is going to see in any Lionel starter set.
Having said that I would use Gargraves/Ross for any public display layout because when properly painted and ballasted it has the best appearance; looking most like real railroad track. Yes, the rail is oversized, the ties are oversized and spaced wrong and there is always that third rail but the mind, (especially an uninitiated mind not knowledgeable on track details) glosses over those details when the track LOOKS right.