This is a non-question for me since Sapphire was the first Pokemon cart I ever actually owned. MUDKIP, hands down. No contest. The other two starters are great and all, I adore all three but me and Mudkip, we go way back.
I love you, Mudkip. I remember the times we trained together, the battles we fought when I was just a kid who was still learning which type was weak against the other type. I remember when I found out that you would only learn Hydro Pump if I didn't let you evolve and we agreed that you wouldn't so we stuck it out together as our team grew and so did our bond. Finally deep into our adventure you mastered the move Hydro Pump and we looked at each other for the last time as Trainer and Mudkip, and then you evolved into Marshtomp. And then even later you evolved into Swampert. We surfed Hoenn's seas together, and I will never forget when we challenged the Elite Four many times. I was just a young kid but you were always there and we never gave up and we learned our opponents and made it all the way to the Champion after many failed attempts. We saw Steven Stone, and we looked at each other one last time just as we did when you finally mastered Hydro Pump before we battled Steven alongside the rest of our team. We won, and it was all because you believed in me. Thank you.
Um, so yeah, I think my favorite is Mudkip. Just a hunch.