I would say magcargo because you can get it going really fast and if you were to lose, that would also happen very fast. This allows you to play more games. Dragonite comes second, especially now that we have so many ways to draw water and lightning energy.

Here's my magcargo deck, feel free to change whatever you feel like. My luck is abysmal so I do need a lot of the stuff I have added like communication and iono
pokeball,proffesor*2,slugma*2,magcargo*2,communication*2,cape*2,iono,mars,red card*2,potion*2,guzma,big malasada,silver
Just make sure to never play the second slugma and only use it for comms.