On Alpha Sapphire, I'm going with Mudkip because it was my starter in Sapphire, I like water starters best/always go with them in my main game of each pair, and Mudkip is my favorite out of those 3. On Omega Ruby, I'm debating between trying Treecko or going with Mudkip again. I do like Mudkip a lot, so it may win out for both files (with the other starters traded over/bred instead).
I'll probably use my starter's mega, unless there's another new mega that I'd rather use.