I'll put my 2 cents into this conversation.
I LOVED Gen. 3. I had over 500 hours of play time in it. And as soon as I heard that D/P Was going out, I freaked out. I obsessed about getting the game for a long long time. In fact, I bought a DS just for Diamond.
To be honest, I think GEN IV Is better. Yes, it may not be as action packed and fast, but it had the best Graphics and gameplay during it's releases out of all the pokemon games. I had about 700 hours of gameplay in my Pokemon Diamonds. It just never got boring, because everytime I'd start getting bored, I'd transfer some pokemon over from FireRed or Emerald and train them to make a new pokemon team. Plus, Shiny hunting often took up most of my time.
Now, I do LOVE Generation 5. You have to admit, it has some kick-butt graphics. Walking over the first bridge for the first time BLEW. MY. MIND. I was like "HOLY MOTHER OF POKEMON, THIS IS AMAZING. I do admit, Gen. 5 has a pretty good map, and after you
"beat the game" You still have a lot to do.
Is it better than Gen. 4? I don't think so.
I believe Gen. 4 had better pokemon. Especially starters. I mean seriously? ANOTHER FIRE/FIGHTING? At least Infernape looked pretty cool. But besides that, Gen. 5 didn't have many pokemon I really liked. I loved Victini, Zorua/Zoroark, and Reshiram/Zekrom. But not many other's appealed to me. In fact, my pokemon team right now doesn't have any Gen. 5 pokemon besides my Serperior.
In conclusion, I think Gen. 4 will always be better to me, at least. I have way more memories, and much more fun playing it, than playing pokemon Black/white. (And i've logged a good 170 hours into black and white so far.)
It's just, I made my bestfriend with Gen 4. I had Diamond, he had Pearl. I had Soul Silver, he had HeartGold. It made Gen. 4 MUCH more fun, to have someone to play it with and experience it with. In my heart, Gen. 4 will always be the best. <3