My opinion remains very separate from generation to generation - each generation has the strong, and the weak.
I always found the first original pokemon games to be kind of a drag to be honest, and played theme merely for the wonderful pokemon designs such as Gyarados (which I oddly don't like that much now.
), Mew, Mewtwo, Kangaskhan, Nidoking, yada yada.
I loved Johto for the overall story, which made me WISH I could be a Team Rocket member, since I adored them at that point, and that's when I saw them develop this very dark team of pokemon like Murkrow, Houndoom, etc. The areas were neat, and Crystal had taken over my life for a very extended period of time. It just felt fun. I really only disliked the fact that the pokemon in it were hard to raise to higher levels due to things being too low-leveled.
I found Hoenn to be even more fun, and while I was devastated that I couldn't get my original Pokemon from my Crystal game, I was entranced at the rich atmosphere, and the extremely frantic island. I could see how that could turn people off, but there was NO boring part about it. Not to mention that I felt this was the most challenging of the games yet. I didn't like the fact that it was absolutely mandatory to surf as much as I had to, which was indeed quite a bit, and the two evil teams were rather eh.... but I liked the overall outcome and self-awareness of both Archie and Maxxie. I felt the pokemon designs were the best of any generation. The battle frontier was also brilliant in Emerald.
The fourth generation, combat wise, was the smoothest. The split between the two factions of physical and special opened new windows of opportunity to many pokemon that previously were hampered by their own type, and now things like Absol, Gengar, Azumarill, Sharpedo and many others, were permitted to fight more freely. Many moves were added that made previously underlooked pokemon worthwhile (Ninetales).
However, to me, as a total, the island had rather ugly colors, and the characters were rather dislikable. I thought that Professor Roan was very intimidating, and nowhere near as approachable as the previous, the Rival was lacking any sense of personality, and felt like Pikachu incarnate in a human - all laugh, all cheekyness, and no substance unlike the yellow favourite rodent of ours. Things didn't feel like they flowed as freely, and often felt more linear than before, and a ton of the new additions made no sense to me, such as the underground (it's ugly, and rather unappealing compared to the secret bases from before. Could have made it cooler looking). The music didn't click with me like it did in Johto and Hoenn, and while I must admit I enjoy the entirety of the story more than ANY other pokemon game (sans colliseum I loved that tale), there simply wasn't as big of a draw for me. I dislike many of the new pokemon, not even compared to the old, but on their own right. I like many of the legendaries, but think there were too many of them, and others simply looked too bright, even surpassing the Johto pokemon. They looked more cartoonish than the previous, and while every generation has had very bright, vibrant pokemon, this one had them in excess. I also felt that once I beat the elite four, things felt very scattered, and incomprehensive. I didn't know what was going on, and things didn't make a ton of sense.
In total, I really do enjoy the fourth generation for some of its addition in pokemon, but mainly I think it shines with its story, the battle mechanics it brought, but I feel that much like the second generation, the focus was on improving past gripes, rather than bringing something new to the table in terms of pokemon. I just think Johto did the pokemon aspect better, while Sinnoh brought the mechanics improvement better. I still say Kanto and Hoenn shine in originality though, whether it was good or bad, it's hard not to admit that those two were very original and unique.