What is the best starting Pokemon in Pokemon ruby?

There really is no best choice, but I usually pick the Mudkip, because it's good against the first gym and it's semi easy to raise. Fire pokémon are hard to train, but really strong when you do train them. Grass is easiest to raise, but I personally don't really like them. It really all depends on what you want and like.

My advice is to build your team around whichever starter you pick to buffer its weaknesses, then you can practically beat anything.