I'd have to say between 1 and 5. Gen 1 was far too glitchy for me (admittedly expected of the first attempt but come on) and really the only reason to ever play it now is for the nostalgia factor, which I never had in the first place. In fact, my only memory that comes to mind immediately of Gen 1 was back when I was bad and got stuck in Seafoam Island and had to start a new game entirely.
For a slightly similar reason, I don't like Gen 5 much either- I just didn't get too attached to it and had more fun playing Blaze Black than I did the main series game. And out of all the versions I've played, I only finished Black and even then did NOTHING with it afterward. Got halfway through Blaze Black and then laptop issues halted that, and I played like 10 minutes (or less) of both White 2 and Volt White 2.