I plan on starting with Treecko when I get Omega Ruby. It was my first ever starter in Ruby (my first ever GBA game) and is the only starter I ever legitimately raised to level 100 in my gen. III games, so it holds a lot of nostalgia for me. I definitely plan on using its Mega; Mega Sceptile looks pretty badass and is pretty much what I hoped for it to be. It's definitely gonna stay on my team. I wouldn't take it out for any other Pokemon--even others with Megas I want to use.
In Alpha Sapphire, I plan on starting with Mudkip. I really love its and Swampert's design & shiny color palette, and I love Swampert's typing and bulk. I also have some good memories of Swampert; the one I have in Emerald has done pretty well in battle, including ones with the Frontier Brains. Like with Treecko, I plan on keeping it in my party; however, I will not be Mega Evolving it. I'm not a huge fan of Mega Swampert's design, plus I've been wanting to use Mega Manectric in-game, which I decided I'd use before Sceptile's and Swampert's Megas were announced. Not gonna wait to use it in another playthrough or in the next game(s).