From first to last, I'd go Johto, Hoenn, Unova, Sinnoh, and then Kanto. Zat's right, America. Come and get me (in a German accent). OK, here are my main reasons:
Johto: Had improved evolutions from Kanto, such as Scizor, Bellossm, and Steelix, while also having amazing original pokemon, with special mention going to Skarmory and Ampharos. The Overworld was also very original and is always a Nurse Joy to explore, while also feeling the most Japanese. And you could even go to Kanto and fight it's Gym Leaders, and then even Red! And while Kanto is pretty much the same, it did make some very noticable changes, like the magnet train and the remains of the Safari Zone, which was thankfully put into Johto in the remakes.
Hoenn: This region had more un-creative designs, but made up for it by adding abilities, the unique pokemon castform, and TWO different bad guy groups. But for some odd reason, they don't use their signature types. Anyways, the overworld was somewhat non-linear (in my eyes), and had included pokemon contests, which aren't as cool as in the anime, but still takes away the stress of battling. The Gym Leaders are very tough now, with a fighting type as the second gym, the electric gym can be a total annoyance unless you choose mudkip and evolve him, and the psychic double battle that's annoying unless you choose Mudkip.
Unova: This region, while it has the fewest creative pokemon, are very good in battle and have such unique typings, such as Ferrothorn and Volcorona. The overworld, while having no access to non-unova pokemon (unless you're going to get celebi and the shiny legendary dogs), is still fun to explore and has tons of hidden items and rare pokemon. The Gym leaders are all great (besides that Spammer Elysia), and their gyms being so enjoyable, with special mention going to Skyla's and Iris'. (I choose White) And every time I see the Vanillite line, I go "Om nom nom"! That kids, was an onomatopoeia, a word that describes a sound. :nerd:
Sinnoh: Sinnoh is probably the best first game for a new pokemon player. Why? Because the game just feels too slow. The pokemon are somewhat creative or just likable, unlike others (Purugly). The overworld probably has the most details, but mainly in the towns, while out in the fields it just feels like I'm replaying Johto, which ironically enough was remade during this Gen. Now, the storyline of this game (more platinum than the others) was incredibily dark, but just felt a little boring to me. However this game, like Unova, had quite a lot of replay value, in which you gain access to a new island where you can catch Hetran and Skarmory.
Kanto: I can just imagine all the fanboys (including my older brother) screaming at me, but this is
my opinion, so just listen. It's not that I hate it, it's just that it was the first game, and still needed a little time to develop, which I think Johto finished. While most of the pokemon appeal to me, others, like Victoribell, Meowth, and Tentacurel I mearly just shrug off. The overworld was pretty bland, but again, this was the first game. The gyms were just about average, but LT Surge's gym was particularly unique, and had an terrible aroma of fish.
All in all, no generation/region of pokemon is bad, it's just a matter of which you like better, and what kind of person you are.