It's a tie between Brawly and Tate and Liza, albeit varying. Everytime I pick Mudkip as my starter, Brawly's Makuhita always beats the heck out of my Marshtomp. It spammed Bulk Up and Arm Thrust, and I can't line up a decent shot due to that raised defense.
And when I picked Torchic, I always had a tough time against Tate and Liza, due to the fact that I'm facing twins, with type advantages no less.
So in total,
Mudkip - Brawly
Torchic - Tate and Liza