I'm not a huge fan of any Hoenn starters, though Mudkip I guess since I've never used it. Blaziken would make things too easy, its Mega Evolution is so OP and XY had a huge habit of handing the player power-ups they would need to kill their opposition with ease. I'm betting Steven Stone will give the player their respective Mega Stone for their starter, Blaziken's Speed Boost and overwhelming power would skewer any competition. I like Sceptile but I've used it often, so Mudkip I guess.
Probably Treecko. I don't care for Mudkip and I already used Torchic in X. Besides, Treecko's a Grass Type. And I love Grass types. That said, I did just play my Gen III games (twice) recently, so I'm a bit tired of all the Hoenn Pokemon. This feeling will probably change by release time, but for now I feel like using my trusty party of Bulbasaur/Dragonite/Scizor/Houndoom/Manectric/Milotic to traverse Hoenn in this journey, so I might just end up boxing Treecko entirely.
It's tough: the Hoenn starters are the best set of starters imo. I've always used mudkip and I'm pretty sure that I would've used torchic once before. But recently treecko's grown on me, to the point where sceptile is one of my favourite starter final evolution (Along with serperior and torterra. Which is odd since I usually don't like grass types.) I can't take a karate fire chicken seriously, not to mention I've never been partial to fighting types.
I've never gone treecko before and Sceptile is now one of my favourite pokemon, so definitely treecko. There's also serious potential for an awesome grass/dragon mega evolution.
I've grown to love Sceptile a lot in the latest years, but 10 years ago I just loved using Swampert in Ruby. I'm probably going with Treecko at first, unless the mega evolutions have an influence on my choosing.