What's your personal favorite chunk of any pokemon game? A lot of people have parts that they don't like (a certain tutorial heavy section in BW2 gets a lot of flack for example), but I rarely hear people talk about the parts that they like. Y'know, like when you decide to do a playthrough it's usually the part that you have in mind when you look forward to playing it.
Anyway, for me it's the stretch between the 1st and 2nd gyms in Platinum, in particular Floaroma town and the surrounding environments are just an absolute joy to experience. I really like how the early game in pokemon usually has a unique kinda lighthearted feel to it, with the pokemon you see mainly being non-threatening first stages. In DPP in particular the music also REALLY helps with this whole vibe, being laid back and jazzy, but also fun and whimsical and adventurous.
https://www.youtube.com/watch?v=-xYovydwZio
https://www.youtube.com/watch?v=O1d5DGiYt20
https://www.youtube.com/watch?v=CyrBuzhLqwM
https://www.youtube.com/watch?v=ZZ6vEvr-NLQ
And then towards the end they cap it off with THE best forest theme, which is suitably mysterious.
https://www.youtube.com/watch?v=Gw8OOD3XcCI
Plus I associate this section with variety since you get access to Buizel, Shellos, Pachirisu, and all the honey tree encounters pretty much all at once.