I've always thought of Palpitoad as being the Kingler of Unova except a bit more used.
I could see Oshawott being his water powerhouse because being the water starter, it promotes the game a lot more then an armless tadpole frog thing ever could. I do see Oshawott still evolving though. As for what Mew~ said, I actually could see Ash or Cilan or Iris catching a Karrablast and trading it for Bianca's Shelmet for evolution. I couldn't see any of Ash's old Pokemon being the star, no matter how much I want Kingler to shine, it won't happen.
I wouldn't exactly equate being a starter to promoting the game necessarily as a powerhouse. Piplup had ample screen time and I never considered it a powerhouse. I could see Oshawott being Piplup 2.0 or a anchor to Ash team. On top of that he already has 2 other starters that can be seen as powerhouses if the writers want to go the route of Piplup 2.0 for Oshawott.
Squirtle - He probably won't been scene until post main story or at the league so I can really see him making an impact-fulness that would make me say he is the default powerhouse at the end of the day. Guy is already a powerhouse in his own right, the way he fought Brandon it is just as strong as any others and any other of Ash water types.
Kingler - I can see him making any more of an impact that would make me think it would be a powerhouse either. He is only used in tournament settings, which begs the question well does that mean he is a powerhouse cause he is used when it matters? Ehhh not really to me, I never saw him in BF so I really couldn't say.
Totodile - Do something to prove you merit being mentioned as a powerful pokemon. I don't remember a battle or an worthwhile battle since Silver Conference.
Corphish - If they bring him back because of RS remake then I can see him making some impact, but until then I really can't see him as the powerhouse by the end of the Saga.
Buizel - If they bring him back he has just a good a chance as any to do something. I'm sure we will get to see him prove his worth again during the Conference in Unova.
Oshawott - He sure is one of the most seen pokemon in the Unova Saga, but will that correlate to him evolving or actually shining in battle? That has yet to be seen. Yes he did beat Palpitoad, but I haven't really seen him do much in terms of battle since. He seems to have a bit of cowardice to him, so I don't know how well that reflects him being a powerhouse... Him being a powerhouse to me is dependent on if he actually evolves to Dewott or Samurott which is what I was hoping for at the beginning of the Saga.
Palpitoad - Since his lost to Oshawott he has done nothing but shine. He did good at Don George Tournament, and from the looks of it he won't do to bad against Elesa Zebrastrika. He could be a go to poke in the big matches and from time to time. If he evolves to Seimitoad that will probably solidify it for me as him being the powerhouse.