I would say, Turtwig. Since it can beat some Gyms. You can pick Chimchar. But in the first Gym, You will have some trouble beating, Roark. Since, Roark has Rock Type Pokemon. But if your Monfernape has Mach Punch, Which it learns when it evolves at Lv.14 and it automatically learns it. But it really depends on you. If you want to beat the Gym Leader fast or take time training. And, For Piplup, I agree that it has great Defense but somehow slow. But it will be a great Water/Steel Pokemon.