Mag Prime
The Pokemon Avenger
None.
Well, jokes aside, it is a popular pokemon and Ash is missing a 6th pokemon.
So the possibilities to consider are:
- He continues with only 5 pokemon. Possibility: Decent. Reception: Worst. (Honestly it would suck to get a league without even a single full battle.)
- Poipole comes back. Possibility: Decent. Reception: Good. (Did Poipole even have a battle while Ash had it? Either way, many people would be ok with this.)
- He brings back a pokemon from Oak such as Charizard. Possibility: Low. Reception: Mixed. (I guess it depends on which pokemon comes back but some people will be pissed, some people will be happy either way.)
- He catches a new pokemon. Possibility: Low. Reception: Good. (I'm sure people wouldn't mind but I'm not sure where he is gonna find a new pokemon halfway through the league.)
- Greninja comes back for the league. Possibility: Lowest. Reception: Best. (Greninja is a popular one right? I don't suppose there are many Greninja haters out there)
- Ash borrows somebody's pokemon. Possibility: Low. Reception: Bad. (I don't expect people will be particularly keen on Ash accepting donations because he couldn't manage to catch a full team this region.)
However, in terms of fan reception, the best case scenario would probably be Greninja, with acceptable possibilities being catching a new pokemon or Poipole coming back. Bringing back an old pokemon could work but I can see half the people raging if Charizard comes back and the other half raging if something other than Charizard comes back, so I think it's a bit risky. I'm sure many people will not be happy if Ash stays with 5 pokemon or borrows a 6th pokemon from somebody else. I sure as hell won't be.
Wouldn't it be cool if they actually gave (me) us what we wanted for a change and brought Greninja back?
tl;dr Realistically though, Poipole strikes a healthy balance of likely and acceptable and so my money would be on that one.