While I'm here, I was thinking about Z-Hydro Cannon Greninja, and how it takes advantage of Torrent to deliver a ridiculous one-time nuke, and it got me wondering if any of the other starters could maybe pull off something similar? Primarina can already with Oceanic Operetta, and I'd say that Grass-types have poor enough coverage that Spectile or something would never work, but do you guys reckon there'd be any merit in a Z-Blast Burn Infernape build? lol probably not but fun to think about
Hey, probably not the best to answer this, but perhaps I can give my perspective to why we only see Greninja using this strategy and why it's likely to stay that way.
First thing to look at is the speed tiers, you need to get off a quick substitute up against most of the tier. I generally tend towards base 100 past which I start to consider mons speedy. That means you are outspeeding Mimikyu, Lando-T, both zard forms (assuming no DD/FC), specs lele, gliscor, volcarona pre-QD, Mega Kangaskan and Zapdos. Of course this isn't ideal, but is what I'd call a bare minimum to be able to substitute before the majority of the metagame and thereby guarantee overgrow/torrent/blaze range.
Of the starters, you have the following above base 100:
Greninja: Base 122 speed
Sceptile: Base 120 speed
Serperior: Base 113 speed
Infernape: Base 108 speed
Delphox: Base 104 speed
You also technically have the elemental apes which hit base 101, but I'll discount these for now.
Ideally, you'd also want to be above base 110, outspeeding Mega Metagross, which would lower the viability of both Infernape and Delphox for this strategy as well.
What makes Greninja so much more suited to it than those other mons?
1. Typing
I'm just going to come out and say that water is the best offensive typing out of the starter three (fire is close tho and this is just my opinion), at least when it comes to BSS. A simple look down the viability rankings and you can see 12/49 mons in S -> A- actually resist water, of which 3 of them like for example Kartana are basically getting OHKOed by Z-Hydro Cannon anyways. 21/49 mons resist grass, sure some of them will fall over anyways like Pheromosa, but in general its just a lot easier to check defensively, some key pokemon like mega mence, mega venusaur, mega zard, celesteela and ferrothorn four times resist grass. Similar story can be said for fire type to a lesser extent, though only 14/49 pokemon actually resist it, so perhaps you could run a delphox or an infernape.
2. Ability
The fact that Greninja is a lot more unpredictable than these mons allows it to be run easier. Yes, this Z-Hydrocannon set is the flavour of the month, but one has to respect protean Greninja, whereas the other mons aren't as offensively threatening. If running this set, I can bring mega mence on every other starter mon without some weird tech except Greninja, which outspeeds and ohkos it if it has ice beam. There is also serp glare, but I could technically risk that and be ok the next turn. Same with speed boost mega blaziken which is 2hkod by priority shuriken. It's that super fast speed tier that really pushes it above the edge, yes sceptile has a very fast speed too but its movepool isn't anywhere near as good. Infernape has a great move pool, but doesn't quite have the speed and why use it when you could use mega blaziken instead. This is the same issue Delphox has, except its also a bad mon on top of that. Notice I haven't mentioned serperior yet, it's fast, it has a great secondary ability contrary that is real threatening too, but its threatening only because of the boosts. Without the boosts, it has worse offensive stats than meganium at base 75. That ain't killing no-one.
Overall I don't think we'll be seeing anything except greninja here and I'd be tempted to say that despite the fact is a nuke, I think it can be played around and is actually quite predictable considering you need three substitutes to get into torrent range, which means its telegraphed quite heavily a lot of the time as Greninja isn't eating neutral hits at all and suffers in coverage since both hydro cannon, shurikan and substitute are needed IMO. Shurikan is essential since it guarantees extra damage that none of the other starters have after the z move and the enemy poke breaks the sub. I'd be tempted to say it'll probably drop off as people learn how to play around it and I find protean to be a lot more scary in what it can do, but that's just my personal opinion and I may well be wrong.