These are probably my favorite Pokemon games ever, and for a number of reasons.
First off, the main campaign. It expands upon the Unova region from the first Black and White with new areas, new Gyms, and expanded Pokedex containing Pokemon from other regions across the series, as opposed to the first BW which only had brand new Pokemon. This adds more variety to team building, and spices up a familiar region with "new" faces to keep things interesting.
These games introduced the Pokemon World Tournament, or PWT for short. In the PWT, you battle in a tournament which includes familiar faces from across the series, such as Champions and Gym Leaders. It's a fun challenge that keeps you coming back for more, and while it may not be expansive as Battle Frontier from previous games, there's still fun to be had here.
Now, I'd like to talk about Gen V (BW, B2W2) as a whole and why I enjoy it so much. The gameplay engine is greatly improved over Gen IV (DP, Platinum, HGSS), with the games being much faster paced both on the overworld and in battle. This makes the game significantly more enjoyable to play, especially for repeat playthroughs, whereas Gen IV was very slow to play with unnecessarily long battle animations, slow text speed and slow movement on the overworld.
Another highlight of Gen V is reusable TMs. A series first, you can now teach your Pokemon moves with TMs as many times as you want, providing much less of a headache trying to decide who to teach a good move too. This vastly improves the playing experience ensures that you can always have optimal moves on your team.
All of this is neatly packaged in a familiar yet fun Pokemon adventure makes for a solid entry that you can't go wrong with. The story does take place after the first Black and White, so I'd recommend playing them first (they're solid games as well), but the story itself isn't as prominent in these games, so it isn't necessary to play BW first unless you want the story to make a little more sense.
Overall, these games are my favorite in the series, and I return to them numerous times and always have a blast. Whether trying out new teams, or randomizing the game for even more fun (and nonsense), Black 2 and White 2 are Pokemon at it's best.