It’s a hard choice between Platinum and either one of HGSS (and which one of them would be another hard choice).
On one hand, Sinnoh is my absolute favourite region and Platinum has one of the best storylines ever; on the other hand, gen 2 is the generation I’m the most nostalgic about, it has many of my favourite Pokémon (though gen 4 also has many) and my favourite set of starters, and it’s also packed full of stuff to do with the two regions. Maybe I’d go with SoulSilver, as despite Gold and HeartGold being my original choices I prefer the exclusive Pokémon SoulSilver has.
For me I think it would have to be HeartGold. Emerald's actually a tad higher on my favorite game list, but HG/SS (and the 4th gen overall) added so many QoL improvements from the previous gen, that I have to pass on Emerald here.
HGSS. Lots of stuff to do - and not just talking about it having two regions in it. Pokeathlon, the Battle Frontier, Gym Rematches/number collective, photos, the Pokewalker... lots of great content, and as I have personal bias (nostalgia) for those games I'd be happy to replay as well. I have played it multiple times, after all. :V