Had objectively the best metagame. Everyone was mostly balanced and fit neatly into tiers. Everything before Gen III was a mess. Gen III was too overly simplistic. Gen V had a terrible power creep that the series will probably never recover from.
Sinnoh had a good overworld that was really fun to run around. Had a variety of terrain, and wasn't just an empty ocean like in RSE or linear snorefest like BW. You traveled through Coronet caves constantly, and it made the whole game feel really connected despite being non-linear.
Had objectively the best postgame. There was a lot of sidequests to do in the story mode, and due to the playable metagame and auto-level 100 wifi battles, there was a LOT to do in battles (there's a reason that people still use gen iv on the competitive scene.)
The story was engaging enough for a pokemon title. Team Galaxy was properly used throughout the game without appearing at the last second (RSE) or just completely interrupting the campaign with a fan-fictiony story that jumped the sharpedo 20 times (BW.)
On a similar note, The climb up Coronet and the battle against the game legend was really climactic. You travel through the main caves constantly through the main game due to the nonlinear nature of the overworld, but near the end you actually get to navigate the upper levels, through the snow, not knowing what awaited you once you reached the summit. the atmosphere was perfect. and dat music: http://www.youtube.com/watch?v=M-GFxbkGmMQ
also, it had HGSS which everyone loved.
No idea why it's so hated. I admit the designs were a bit lackluster, but you're a complete moron if that's what you base the quality of a generation off of.
I'll give Gen IV this, it had the best starters (apart from Infernape we actually got some really cool new dual typings this time around), the best champion, the physical/special split was long overdue, it took the series online and Platinum gave you early, easy access to awesome Pokemon like Eevee and Porygon. That said, it was slow as ****, Diamond & Pearl especially were full of obvious flaws, the ridiculous overuse of HMs made the world anything but fun to traverse and a lot of the new Pokemon were pretty crappy (Wormadam anyone?).
I'd personally rank it second to last among the Gens, only beating out Gen 2.