Games: Diamond, Pearl and Platinum
To many this is one of, if not the, best generations of Pokemon, and for good reason. To kick things off this Gen introduced one of the absolute biggest game changers since the beginning: the Physical/Special split. From Gens I-III attacks were defined by their type, not by stat. For example, Gengar's appeal is it's high Special Attack stat, but if it tries to get STAB (Same Type Attack Bonus) from a Ghost move, it would always be a physical move. Now, all Pokemon can focus on the moves that will actually benefit them in battle, and the metagame has been completely revolutionized.
As for the Pokemon themselves, many fans were disappointed that a lot of the Sinnoh (this region) Pokemon were in some way related to previous gen Pokemon. I for one, don't argue with it. Some of my favorites came out of this batch. Unfortunately, this gen does have some of the more forgettable 'mons.
Story-wise I felt the plot was a bit too similar to the Magma/Aqua story of Gen III. Cyrus (the leader of the villainous Team Galactic) wants to reshape the world in his own way, devoid of emotion. To this end he attempts to capture the legendary Dialga or Palkia (depending on your version). In Platinum (the only one I played) Giratina shows up and drags Cyrus into the Distortion World and the player is forced to set things straight.
As for the location, I enjoyed Sinnoh. I felt that there was enough to explore that it didn't get too boring, there wasn't a ton of backtracking, and it had a good variety of environments. The Gym Leaders were relatively memorable, and as I mentioned in my top battles list Cynthia is my favorite Champion.
If I was including remakes, this Gen would probably hit the top or pretty close. I had stopped playing the Pokemon games when this one came out, but upon learning that they were remaking Gen II I bought a DS and Platinum to give it a try. I've been playing again ever since.
Favorite Pokemon from this Generation:
Gallade, Torterra, Garchomp, Lucario, Infernape, Mamoswine,Togekiss, and the mighty Giratina.
Team I used my first playthrough:
Torterra, Luxray, Staraptor, Roserade, Garchomp, Lucario