Depends on what you can get and how willing you are to put up with the growing pains the series has since it started on the original Playstation. You don't have to worry about any story connecting them, as they're all pretty self-contained; so it's not like jumping from one to another is gonna have you miss out or spoil something.
That being said; I found the PS2 games to be the best, especially the original AC2; but any of them would be a good place to start. AC2, Last Raven, and Nine Breaker are like my top 3, and all PS2. The 360/PS3 games (AC4, Silent Line For Answer, AC5, and Verdict Day) may be easier to find; but they have things I'm not too happy with. Especially ACV, as that game got gimped hard (like 40%+ of content is gone) when the online servers were turned off. So, unless you're willing to understand you're playing an incomplete product, skip ACV completely. I'd be lying if I said I didn't like the ideas implemented in AC5; but its reliance of other humans, online play, and time/day sensitive things really hampereds enjoyment of it. Then when the servers got turned off, it took about half the entire fucking game with it.
One last thing; I'm not too keen on emulation so not sure if it works the same there. But about half the games function like an expansion of the previous one. So if you have save data from the previous entry, you can import your data and start with most (if not all) of the stuff you had.