Pokémon Go's 'Mega Ascension' event builds up to the Go Fest: Mega Finale at the weekend with a choice - Mega Chesnaught, Mega Delphox or Mega Greninja?
Starting on Monday, 31st August, a Choose a Path Timed Research quest opens with a choice between the three Kalos starters. Unlike most branching Timed Research quests where the outcome often doesn't matter, here you are given enough Mega Energy to Mega Evolve only that choice this week. For now, anyway - they'll all be available in a future event of course.
But even if your chance to Mega Evolve the others eventually will arrive, it certainly adds weight to the decision. As for what to pick, here's some factors to consider between Mega Chesnaught, Mega Delphox and Mega Greninja in Pokémon Go.
Mega Chesnaught, Delphox or Greninja Choose a Path: Which is better?
In short, all three Kalos Mega Evolutions are very powerful in their respective types:
- Chespin's final evolution, Mega Chesnaught, is in the top three best Grass-type attackers, behind Kartana and Mega Sceptile
- Fennekin's final evolution, Mega Delphox, is the best Fire-type attacker full stop
- Froakie's final evolution, Mega Greninja, is the best Water-type attacker apart from Primal Kyogre
With all being very good, it's best to consider what would benefit your current collection (i.e. are you lacking a decent Grass-, Water-, or Fire-type attacker?).
Otherwise, if we're going by what tops their typing tables, Mega Delphox is the strongest Mega of the bunch - and there won't be anything beating it in the Fire-type stakes for the foreseeable, either (not even Mega Chandelure arriving in next season's Twilight Trails).

Mega Delphox is also going to be the first Fire-type that can reach Mega Level 4, if that makes any difference - especially when you consider that with the Deluxe version of the event Go Pass, you get enough Mega Energy to Mega Evolve your choice to Mega Level 4. (A little pay to win perhaps, but it'll at least save you grinding a load of raids or walking one as your Buddy for Energy down the line.)
But since they're all strong, you can't really go wrong here, and you should really just opt for whatever jumps out to you. Personally, as someone who loves Greninja, Froakie is likely going to be my pick - though the lure of a very powerful Mega Delphox may sway me on the day.

Finally, outside of the Mega Evolution you end with, your choice also sees an increased shiny chance for Chespin, Froakie or Fennekin respectively, and in the Go Pass, see encounters with that Pokémon have a Special Background plus bonus Candy and XL Candy in the Go Pass. The Deluxe track will also feature an avatar T-shirt unique to your choice, too.
If that matters to you - i.e. you want, say, a Chesnaught T-shirt for your avatar - then have that as a deciding factor as well.

Best of luck with making your choice - as mentioned, you can't go far wrong. Also this week is a refreshed Go Fest: Mega Finale raid schedule if you're after new Megas to catch - as well as the return of Armored Mewtwo.