HG/SS are an easy win for me. D/P/Pt, while I still think are amazing games, are near the bottom of my list for main series Pokemon games, really only beating Yellow.
The below is all my opinion, keep in mind.
HG/SS:
- Better designed gyms
- More memorable characters (Gym Leaders, Silver, Lance...)
- Best final fight in the entire franchise (Red)
- Better selection of Pokemon (The only Gen IV mons I love are the new Evos, which don't come until post-game)
- Auto-pilot run
- Love being able to walk with my Pokemon
- Best variety of side activities in the franchise, including:
- Bug Catching Contest
- Voltorb Flip
- Pokeathalon
D/P/Pt:
- Best legendary encounters
- Coolest use of graphics (Platinum mainly)
Those are the areas that each set of games has that set them apart from other main-series Pokemon games. Again, though, all five games hold up to the amazing quality that we've come to expect from the franchise. For me, however, it's no contest that HG/SS are vastly superior.