Platinum for me. It's still my fave game ever since it came out. The story was good, I played it over and over again. Sinnoh became my favorite region after I finished Platinum, and it's the only game where I felt compelled to do all the extra stuff I didn't have to. I've explained my love for Platinum version in a bunch of threads but yeah it's not just my fave gen 4 game, but my fave Pokemon game to date. HGSS is good, but even with all it's improvements it doesn't even touch Platinum version to me.
Parena or Serecham both equal Serena+Pancham!