Gen 3 and 4 are the most polarizing of the series. Gen 1 and 2 are loved by people who grew up with Pokemon everywhere, while Gen 5 is hailed for being a revolution for the series (which it definitely wasn't) and bringing back many fans who heard something or the other like that. Call me a hipster, but Gen 3 and 4 are my favourites, with Gen 1, 5 and 2 behind in that order.
First off, I'm mainly reviewing Platinum because its the definitive version, as it fixed everything wrong with Diamond/Pearl. Few Fire-types? Fixed. 151 pre-Elite 4, regional 'mons not enough for you? Bam, now there are 210. Pokémon/trainers encounter animation too slow? Now faster.
Sinnoh was a great region, second only to Hoenn in diversity. The atmosphere was awesome and it gave me the biggest 'feeling of adventure' of the Pokémon games. It had some really cool ideas, like a city focused on solar energy and a huge mountain range dividing the region, and three lakes of legend. The Sinnoh mythology was intricate and it was great how each legendary played a role in it.
The huge change this generation was the Physical/Special split. Moves are no longer categorized Phy/Spe wise depending on type, but rather on their nature. This means I can actually use Gyarados as a Water-type with moves like Waterfall being physical, as opposed to all Water moves being Special and Gyarados being limited by that due to low Sp. Atk. I can't stress how great this feature is.
Kanto and Johto have memorable 8-bit sound (FR/LG had a great soundfont but HG/SS was okay) and Hoenn has catchy and epic trumpet music limited slightly only by GBA sound quality. But Sinnoh has that sexy piano. The music is diverse as it ranges from adventurous (Route 205) to soothing (Route 209) and even jazzy (228)! Day/Night music is great and they sound distinctly different, unlike Black/White where the seasonal change in music is limited to things like simply adding some Christmas chimes in Winter, etc.
The difficulty, battle-wise, is higher than previous games (see entry on Sequel Difficulty Spike
for details) and this is fantastic! More and more trainers/gym leaders/Elite 4 members had a diverse party and gave moves to their Pokemon which countered their weaknesses. It honestly made the game much more fun, as (lack of) difficulty was one of the reasons that spoiled Johto games for me.
Questions are welcome.