I can agree with B maybe. Shroomish does nicely against Roxanne. It doesn't have a great moment with Brawly but it does nicely within the beach since there are a lot of Water-types (or Azurill which does crap to Shroomish). Once its Breloom a lot of its issues are fixed. Mauville is a good time for it too because it has access to Low Sweep and Power-up Punch. Then you have Rock Tomb from Roxanne too. Only things that suck are Seed Bomb is late (Level 44) and Sky Uppercut being one of its stronger moves (Level 39).
It definitely seems like it's somewhere in limbo between A and B. Having only Mach Punch and Force Palm for STAB until fairly late (say gym 6-ish) definitely starts to hurt it, though it still gets decent coverage with Rock Tomb from 1st gym, though this also drops off in power until you get a stronger Rock attack (I forget where Rock Slide is and Stone Edge isn't great in-game due to low pp and accuracy). I don't feel extremely strongly one way or the other. I would urge us all to be conservative in our decisions though, since greater stratification on the top end is far more useful than stratification on the lower end (where people aren't going to be looking as often, and, thus, delineations need not be as stringent).
Obviously this would have to be expanded more. For example optional DexNav moves are good for Breloom - Wake-up Slap, Bullet Seed, Seed Bomb, and Drain Punch are nice for Breloom. It arguably has one of the better types against Team Aqua too (Grimer / Zubat suck but there are workarounds for Zubat).
Of course. I didn't intend for this to be included in the OP, just as a quick way of glancing at it's strengths an weaknesses. I apologize for that and can make a more detailed one if needed (though if anyone else wants to do so, they should feel free).
Also I really think Treecko should be at the least an A.
I know I may be seen as crazy but we have to remember that Treecko has almost auto-win capability with Gym 1 on Mega Drain alone and potential evolution. Brawly is eh, but Grovyle has access to Low Sweep with Wattson and a resistance to Volt Switch. It can sort of dent Flannery since Rock Tomb is available to Grovyle (as is Dig) and it should get through most of the Fire-types barring Torkoal and -maybe- Numel. It should do okay versus Winona too since, despite the Flying-type weakness, it can get through Pelipper without too much issue and Altaria is prone to Mega Sceptile's Dual Chop.
Honestly it isn't the strongest Pokemon and I can see why questioning it in S exists, but it being in B is pretty damn insulting too. Treecko has a couple hiccups along the ride but it is damn strong and has a versatile movepool to work around some of its issues.
I can definitely see A. It runs into issues that are definitely less prevalent with the other starters, but it's perfectly strong in its own right too, and it's viable movepool has been expanded greatly since RSE.
 
 
		
 
 
		




 
 
		 
 
		

 
 
		 
 
		 
 
		
 
 
		 
 
		


 
 
		 
 
		
 
 
		 
 
		 
 
		
 
 
		 
 
		





 
 
		