I also really like Piplup, and if I had to pick a favorite Pokemon of all time, it would have to be him. While BDSP wasn’t great, it was still the first Pokemon game I really remember enjoying!
My personal favorite Pokemon is really any of the water starters from gens 1-5 and maybe Scarlet/Violet as well because thy all had really cool designs and I liked how they all came in handy at one point in the game or another!
