People seem to be forgetting that the originals were not made in the same way as more modern games. They were made for the Game Boy, as a start to a series. They excelled in this. Then, second generation came along and improved on it (better balance, more Pokemon to choose from).
I'm not saying either of them are flawless (Second Generation watered down Kanto, first generation had overpowered Psychic type). I'm just saying I don't think it's fair to compare them to games like Ruby and Sapphire or Diamond and Pearl when the system and expectations of the audience has changed so much. The Game Boy color games were clearly made in different eras, and they did what they were supposed to do.
That being said, my least favorites are Black and White. I like to think of Black and White as the Shadow Dragon (reference to a Fire Emblem game that had old mechanics on a new system) of the series. It went back to the old system (having only around 150 Pokemon, shorter region, less side quests) in order to appeal to their longtime fans. For some, it worked, since there is a nostalgic feeling in this what the same time having a new coating (this is exactly what happened to Shadow Dragon in Japan, where the series had most of its fans). For others, it failed miserably because they enjoyed the newer additions and wanted something more (this is what happened to Shadow Dragon in the United States, because most of those fans started with newer games and never played the original of the game was based off of).
For me, I am in the second camp. It's odd, because I started with Gold, and got Blue shortly after. Therefore, I have played the older games and know exactly where everything is coming from. Unfortunately, I like new additions so much that when most of it was removed or tweaked, it felt like a step backward (I hated starting with 156 Pokemon when I was used to choosing from around 200 at the start, for instance). When the sequels came out, I enjoyed them, because they had the appeal of the older games while still keeping the newer features (for instance, a massive amount of side quests, but still a fairly short story). --- You know you read too much when you dream about flying books.
Objectively RBY were unbalanced due to keeping Special attack and defense as one stat and lacked the fleshing out and work that made the other games good (ghost-type was completely useless for example). I don't think it was bad, or even the least enjoyable, but certainly the worst. --- If you love a person, it doesn't matter if they cheat. If you love a person, you won't cheat.
Diamond, Pearl, and Platinum. The main villains were awful, the pokemon were bland(Sans Infernape), and it was the first game where the Legendary pokemon hurt the storyline more than they helped it. There was also the utterly confusing, and ridiculous Victory Road that was by far the longest out of all the games, and required the most HM Usage.
There was just so much bad about DPP. Hoenn is a close second though...all that surfing...
from JUST a gameplay standpoint, obviously Gen 1 is the worst since the games get better with each release, but gen 1 is still my favorite set of pokemon --- Rainbow Dash is best pony. The only argument I will accept is for Twilight Sparkle.