Thinking it over, I would say HGSS. It's easy to get consumed in what it has to offer, has lots of intense fights (Whitney, Clair, Lance...) and fun side quests and what not (Game Corner, Pokéathlon, farming berries with the pots, etcetera). And it has what rivals BW2 for the best post-game in the series (same Battle Frontier as Platinum, Kanto, a Safari Zone with 12 areas and object arrangement, and much more). There's also the touch screen menu which is really nice.
I do like Platinum though, but nowadays i've been enjoying HGSS more. (More than I thought I did for a while, haha, this is coming from someone who GSC and RSE are the least favorites of.) However, HGSS stinks big time for training Pokémon, while Platinum has the Vs. Seeker, although these are just my preferences.