Heartgold and Soulsilver feels like the peak of the series because it has so many cool features like custom safari zones and the Pokeathalon, it has a good system to rematch gym leaders, mystery events like the Celeby, fun side quests like Suicune, and it promotes trade by getting a Hoenn legendary in one game and getting you to trade in order to get Rayquaza.
There is just so much for you to expeience that it is hard to sum it up in one article.
Before Heartgold and Soulsilver
The PWT is awesome and it was the first game to get me into the metagame. Plus all of legendaries to catch throughout that make it worth playing just for that. This is also one of the Pokémon games with the best music. Not only do all of our favorite themes but you have a lot of new music as well.
Plus I feel that they still have the best story of all the games.