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I'd go for Turtwig or Chimchar for ingame.
Turtwig gets really good and varied moves (Crunch, Giga Drain, Earthquake in mind), Chimchar's line becomes really good once it gets Close Combat.
As far as I remember, Piplup's movepool is straight garbage. For exemple, Torterra and Infernape learn Earthquake/Close Combat at level ~40 or less, meanwhile Empoleon gets Aqua-Jet and Whirpool around the same time. Also, you won't get any Steel type move besides Metal Claw from leveling. Don't be afraid to use your Flash Cannon TM on it if you choose Piplup.