In all my Hoenn games (I have 5 copies - 3 of AS and 2 of OR) all times I picked Mudkip. Only on my first run did I keep it in my team. The other 4 times I already had my team planned out.
As for the other 2? - Treecko I plan on using someday in a Grass-type team, I hope I enjoy it - and the other worst looking, nope, won't give it a chance, still don't like it's design, it prevents me from giving it the chance, taking it down so many times in Doubles, just makes me annoy seeing it.
Design, I like Mudkip and it's evolutions a bit more. After using it in a team, and an awesome looking Mega. It's my choice.