The Most Realistic Sounding?

Realistic orchestral mock-ups are not the product of who's got the best samples, but who knows how to use them properly. Which is a very small subset of the population using them, by the way. All of the above companies make excellent products. You can achieve great results with either of them, even better results with several in combination, and the very best results with a solid understanding of how to assemble a good mockup.

The good news is that it's definitely possible, with today's technology, to create orchestral mock-ups that will convince all but the most discerning listeners. The bad news is that it takes years of practise in a number of skills that have nothing whatsoever to do with composition in order to get to the point where you can achieve that.

So if I'm reading your question correctly, you were hoping someone would say "IT'S LIBRARY X!" and you'd say "THAT'S SWELL, MR. FRIENDLY!" and then toddle off to the retailer to buy the thing, and then you'd come home and plug it into your software and suddenly your work would sound realistic. Unfortunately, that's a dream that many folks have and we're not yet at the point where such a thing is possible, for any amount of money (unless you take the approach of paying someone else who's already got the samples and skills to do the mockups for you).

If we knew a bit more about how you work and what your goals are, it would be far easier to give you a useful roadmap of possibilities for how you might go about achieving those goals, but given the nature of your question, all we can offer is a reality check and a query for more information — so how do you produce your music now? What is your goal? Timeframe? Budget?