I think that HeartGold (and SoulSilver) were the best games of the 4th generation- both because they could be enjoyed as a nostalgic experience for older players, but as a brand new game for younger ones, as well.
Plus, there were so many little things that they did well that really made the game a fun and unique experience. We had the opportunity to have a Pokemon always follow us around for the first time since Yellow, we were given a brand new side game in the Pokeathlon (which I'd say is much more fun to play than the Super Contests of DPPt), we could travel across not just one region, but two, and much more. Heck, I was so excited for HeartGold that I actually went out and bought a DS, and I was glad that it didn't disappoint.