I wouldn't say "the best" but Serbia has some comparative advantages for startups. Personally, I own Price2Spy 2.0 and recently started working on botmenot.com, and in my experience, the biggest advantage here is a very professional and educated workforce (developers, marketing specialists, etc.) for a reasonable salary (much lower than in the USA).
I believe India is one of the fastest emerging countries taking part in startups. And yeah the USA has a good market in terms of customers but after going digital initial setup doesn't matter but the policies of that country matters the most.
Depend on your product or service. Quite honest internet has already break the rules and this wall of cities or country is been vanished off.It does not mater now a days. Wherever you sit you can grow your startup anywhere in the world.If you sit in USA if your product is not fit it wont work. Even if you based in Israel or even in third world countries does not matter if your product solves problem you will get your pie of success.