Kalos has a chance to be his best ever - it really depends how he fills out the team. Goodra, Hawlucha, Talonflame and Greninja is an *amazing* first 4. We expect him to get a grass-type to round off the team, right? But what grass-type? I think him having one of the ghost-grass types would be a bit odd somehow. Gogoat is a possibility. But apart from the Chesnaught family that's literally it for grass-types in XY, so maybe he won't.
What he COULD do is get a Florges, which has many grass-type moves in its repertoire. That would be a really nice way to round out his team. I think it'd be good for him to round it out with a Fairy-type too. He could maybe capture it as a Floette, and if AZ is involved in the story that could be a potential device in a subplot.
Earlier somebody mentioned megas - think that would be pretty good but it needs to be the right Pokemon. Personally I'm in favour of him bringing back the Zard and Megavolving him. And because Megazard is an XY Pokemon, that would count as being a member of his Kalos team. I think that is possibly why Zard was bought back in Unova, to remind younger viewers of his existence so if Ash brings him back in XY, he won't be as confusing to them. That said, it'd also be nice if one of his other Pokemon that could mega-evolve got the spotlight, like Sceptile, Heracross or even Glalie.
Or the longshot to end all longshot - the glorious return of Pidgeot, complete with mega-evolution.
So yeah, Kalos team is going to be pretty great even if the final Pokemon is, like, Slurpuff. But as it stands right now, it's not as good as some of the others. Let's have a look then.
His Orange League team. Charizard. Snorlax. Pikachu. Bulbasaur. Squirtle. Lapras.
Lapras is pretty mediocre for me and was just a utility mon to get them from place to place, but Charizard/Snorlax/Pikachu are his big 3, and Bulbasaur/Squirtle are just historically awesome. Tough to beat.
JohtoHOHOHOHOHO not even going to bother.
Hoenn/Kanto Battle Frontier: Glalie. Swellow. Sceptile. Torkoal. Pikachu. Corphish.
The writers made a key mistake in not having Ash capture an extra Pokemon. If you're going to do a "fresh start" and have Ash only use Pokemon from the new region, great - but give him some scope to change his team a little like he did in BW.
Anyway, this team is fine. I'm a fan of it, and because of the restricted numbers they all did pretty great at the league. Sceptile is effortlessly cool. Good team.
Sinnoh team:
Infernape. Buizel. Staraptor. Torterra. Pikachu. Gliscor. Gible.
The late addition of Gible was refreshing, but I'll always remember the other 6 for being a part of what can only be the best battle ever in the series - Ash vs. Paul at the league.
Let's make one thing clear - for me, Charizard is Ash's coolest and best Pokemon. But even Charizard's story can't compare to the depth and the narrative satisfaction provided by Ash's Infernape. In raw storytelling terms, Infernape is Ash's best Pokemon and quite frankly it's barely even close. Charizard wins on cool factor and nostalgia factor, but in terms of enjoying the narrative of the show, Infernape is just so. freaking. awesome. And then you have the others - Staraptor, with that awesome comb and Close Combat. Buizel and his cool attitude...and that cool Ice Punch, especially when he KO'd Gastrodon. Torterra was a disappointment but it's just cool that Ash has one. Gliscor KO'd Drapion and again looks like a boss.
For me, Sinnoh is his best team.
Unova team:
Oshawott, Krookodile, Snivy and a bunch of other scrubs absolutely nobody cares about
Bad. Though maybe that awful league just leaves a nasty taste in my mouth.