I had the most fun with Heartgold because it had all of the epicness of Crystal and even more.
1. Two regions! I like how Mt. Silver was expanded and unlike the stripped down Kanto in GSC, they were able to include an almost full Kanto, complete with a Safari Zone (I know, technically it was Pal Park, but yeah.), and all of the sealed off places, like the Pewter museum of science (no matter if it is smaller ), Cerulean Cave, Seafoam Islands, etc. I just wish they could have put in a full sized Mt Moon and just tacked on Mt. Moon Square in on the top...
2. Same plot from GSC! It kept what made GSC great while adding to it. You can even catch both Lugia and Ho-oh, both of which are optional. It also kept the Crystal sidequest about the Unown
3. The new features! While I don't like the useless walking Pokemon one, I do like all of the new things included such as the Pokeathlon, The Johto Safari Zone, Sinjoh Ruins, the cave where you catch Groudon/Kyogre and Rayquaza, and the time-travel one where you meet Giovanni. Not to mention the "new" features in Kanto. These aren't technically new, because they were in RBY/FRLG, but they are new to GSC/HGSS. They are the ability to catch Mewtwo, Moltres, Zapdos, Articuno, etc.