I chose Turtwig, my first time ever playing. Though, can’t remember which game I got first (Diamond or Pearl). I’d wound up choosing Empoleon, for the opposite version, which is still my favorite water starter and one of my favorite ‘mons.
Around the time I first got Platinum, I chose Chimchar.
I usually went with Chimchar but when I restarted Platinum version a couple years ago I went with Piplup instead just because I was curious to see how good its evolved forms were. I never liked Turtwig.
In my Platinum playthrough, I chose Chimchar (he's obviously now an Infernape). I usually choose the Fire starter and have done so in every generation since Gen 3. I think all the Sinnoh starters are cute, but Chimchar is my favourite.
Piplup in Pearl and Platinum because it came in handy at a lot of gyms in Sinnoh especially once it evolves into Empoleon and gains a wider move pool. I loved using Empoleon as my main battler in Pearl.
Turtwig all the way! Though I really like Chimchar’s line and Piplup’s line isn’t that bad, I have always loved this massive continental tortoise. It tends to fight with Venusaur for my #1 starter Pokémon spot.