IMO:
II>I=V>III>IV
However, I dislike ranking them like this, because I love all of them! I guess if you had to classify me into one of Basilius's categories, it would be a mix of "truegenner" and "sinnoh hater", though I don't actually hate Sinnoh.
I do think Sinnoh had more flaws than the rest of the gens, especially given the fact that it's newer so there's less excuse for messing up because Game Freak should have known what they were doing by the time 4th gen rolled around. Mt Coronet was extremely annoying. I hated having to go through that same stupid boring mountain a million effing times. I also didn't like the complete lack of fire types. Surfing in Sinnoh was pretty much the worst-- SOO slow!! And all those stupid prevos and evos to already-existing Pokemon drove me nuts. OH and the fog part was awful. Defog was the worst idea for an HM ever.
That being said, I did like some things about Sinnoh, too! I enjoyed the updated graphics, I liked Dawn, I adored the starters. The gyms and gym leaders were relatively memorable, and I really loved the snow routes. The mythology was really cool too. And the Underground was so much fun! I loved the secret base feature down there (not as much as the 3rd gen secret bases, but still!), and the digging minigames were also lots of fun.
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I don't really think Gen III deserves all the love it's getting ITT. Sure, secret bases were awesome, and the towns and gyms were relatively memorable, and it had cute starters, but that's about all it has going for it. Team Aqua and Team Magma are really dumb... their goals don't even make any real sense. Expand the land mass or expand the sea? Sure, but... why? It's just pretty pointless. Also, there were no good electric types (Plusle + Minun can go kill themselves, and Electrike was only slightly better). The music was terribad (oh hey let's just blast synthetic trumpets until your ears bleed!) with few exceptions (okay, Verdanturf Town's music WAS gorgeous, and Fortree City's music had a ton of personality). I didn't really like this region's mons in general, either. OH and the rival was really stupid. WAYYY too nice to you, and didn't even bother to fully evolve their starter. I want a rival who is actually somewhat challenging, thank you very much. Still, I like it better than 4th gen because at least the landscape is varied.
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Gen I is where it all started, and it will always hold a special place in my heart. The graphics may have been rudimentary, but that just allowed imagination to run wild. The glitches were really fun to exploit, too, and the fact that they were there made the possibilities seem endless for the Gen I world. The Pokemon themselves were all really great, and although the sprites were kinda messed up, this gave the game some real personality. The music was also really wonderful, especially given the limited hardware GameFreak had to work with at the time. And it had the best rival of all time, Gary Motherfucking Oak. Seriously.
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Gen II had all the benefits of Gen I, except with more Pokemon and more awesome features. OH and you could also go visit Kanto, so it was like two games in one. Say whatever you want about the terrible level curve, but the addition of Kanto in the Gen II games was amazing. It was post E-4, so it was technically post-game anyway, and honestly, no other gen gave us a post-game nearly this rich (well, HGSS did, but that's just because it's a gen II remake, so yeah. I'm not counting remakes in my assessment of gens, btw, or else I'd have to say that Gen IV was the best because of HGSS, but aggghh I do not want to say gen IV was the best because DPP were pretty much the worst). I loved the fact that you could re-battle trainers with phone-calls too, especially in Crystal where they sometimes gave you rare items. The music this gen was my favorite of any gen, even more than gen I, which is difficult to beat IMO. I also loved the plot. And all the areas in Johto to explore. And pretty much everything. The Ruins of Alph in Crystal were the coolest thing ever, and Lugia and Ho-oh were boss. Gen II is the best gen, hands down.
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Gen V is definitely the best in terms of graphics, and arguably the best in terms of story line. I also loved almost all the mons introduced this gen, except for the Swoobat line. Unova was a really cool region with tons of unique locations, and B2W2 only expanded upon this. The only complaint I have about Unova is that it's too linear, but whatever. Surfing is fast again and there's the Dream World. N is an extremely well-written character. The gym leaders have compelling back-stories and insanely cool gyms. Life is good.