Pokemon Black is my game. Since the first ones got little live here, someone has got to stand up for them. Warning: Wall of Text Ahead
Black/ White was intriguing and unusual right from the get-go with the opening theme utilizing more of a narrative structure than you would typically see in a Pokemon intro. The pacing and overall storytelling is much tighter than previous gens, the slower-moving world of Sinnoh comes to mind by contrast. Black/ White doesn't let you rest-- from the mischief going on the moment you set foot in Accumula Town as the radical idea of Pokemon Liberation spreads through the city, to the reenactment of an ancient myth in modern times as you duke it out with the King of Team Plasma's legendary pokemon.
Black 2/ White 2's post game is great and more comprehensive. However, I would have rather seen some of the cool new places and features like the battle tree, nature preserve, world tournament and join avenue incorporated into a third version game along the lines of crystal, emerald, preserving the story of the originals.
I like that Game Freak spared no expense and was really ambitious, but Black 2/White 2 has a tendency to undermine some of the best things the original had going for it. It started with taking a cool Pokemon like Kyurem who did deserve a buff in strength and more storyline-- then over-designing the guy with the fusion forms.
The sequels also don't have as much direction immediately. There are a lot of things you can do like go to pokestar studios that can be enjoyable, but don't contribute to the story. N's prolonged absence also takes a toll on the pacing, since they didn't have another interesting character in an active role for some time.
I don't think that the one-track Hugh was a good character to carry most of the story in Black 2/ White 2, especially coming behind N who was a rival in all but name. Hugh is just way too unevolved with you. I don't even like the idea of his character. Instead of making him look for his sister's lost Pokemon it should have been his own Pokemon.
I did like the introduction of Colress in BW2. He's more of an interesting guy on the edges of the story, than someone like N who sets the story in motion. However, I think they made a believable character, neither a good guy or a bad guy, just a scientist's unbridled curiosity without any apology for it, great battle theme. I just wish the sequels had more gray areas like this.
What was cool in the initial games was that Team Plasma left a little more room for ambiguity. Unlike past teams who nobody would want to succeed, Ghetsis was making a logical arguement about problematic elements of the pokemon world. He was a brilliant, popular man with a silver tongue, preaching about equality. Obviously he had a hidden agenda, but that was the key-- hidden.
I think the sequel jumped the shark with Ghetsis by making him no longer capable of using any psychology to gain power. In BW2 the guy is just raving. presenting the New Team Plasma as a terrorist organization with no pretense to be otherwise. This wasn't improvement over the original-- sorry, its just makes the story more one-dimensional.
To end on a positive note, I did enjoy seeing how lot of the characters were growing like Rood, Iris, Alder, Bianca. Often they were small, subtle changes, but nice. At the end of the day though it was the first game that made me care about all of these characters to begin with. There are some things I like about BW2, but ultimately it is the original that I see as the definitive version of Unova. Gen 5 is one of my favorite gens, but I would not feel as strongly if Black/White had never been part of it.