Ok, I'll cut jokes about power levels....
I'm impressed by so much Snorlax. I think it's a decent and strong Pokémon but I wouldn't pick it for an all star Ash team. Ok, I'll be short since people covered most of what I'd say:
Pikachu: For all reasons given. It's the pokémon who has the strongest bond to Ash. While it's true M20 is an AU, it's impressive a Pikachu - on the beggining of his carrer - could match Marshadow in raw power. Specially when it's implied Marshadow was using his Z-move. Trailers for M21 also suggested he was going one-on-one with Zeraora. Assuming it's basically the same Pikachu we see in the show with some differences on how his story played out, Pikachu's cute look basically hides a force to be reckoned. Prime!universe Pikachu is no pushover since it has beaten a Regice, tied with a Latios and took down TWO pseudo-legendaries on Kalos finals. I've always that had Metagross not hit him so hard, Pikachu would probably soften Charizard up more for Greninja to finish the job.
Charizard: the big lizard might be over-hyped - I like him, to be honest - but it's very clear it's one of the most dependable Pokémon on Ash's parties. Considering movie 3, it's been showed to travel a long way to aid his trainer on a tough spot. And considering their early story, it's impressive Charizard would reach this point for Ash. And it hold his own fairly well against Entei. Later battles proved his might.
Sceptile: Treecko went a long way from being a loner to becoming a very trustable member of Ash's team. The fact so many people pick him just shows how far he went. He came in the show after so many years of Charizard hype and nostalgia, yet managed to be an impressive and powerful character in his own right. His victory over Darkrai is impressive because he just came back after a long, long time in reserve status. Makes me think Sawyer would have some trouble if Ash had his Sceptile even if with mega evolution.
Greninja: power up or not, release status or not, it's still a force to be considered. Much like the early 3 above, it build a powerful bond with Ash. The flashback during Olympia's debut episode showed the reasons why Battle Bond is so rare: it's an occurance where both trainer and Pokémon are completly compatible. Froakie has the same passion for battling, training and getting stronger as Ash. I think there are more Froakies out in the anime world with Battle Bond but because of the differences on their views on battling and personality, it's impossible to draw the power of the alternate form out.
Infernape: super blaze might come accross as glorified cheating but infernape is another pokémon who went a long way. His constant need of proving himself and all the struggle to get over his own traumas made the wimp Chimchar become a competent battler. And without relying on his ability.
Krookodile: from a stalker who randomly created a rivalry for Pikachu - really where did it come from? It wasn't even hinted when it was introduced as a Sandile - it become a good battler. Snorlax is probably stronger but I feel Krook has a lot of unexplored potential.
It's sad Gible couldn't go all way evolution-wise otherwise, Garchomp would be in the final place hehe
and one of those having a quad type advantage over him and being basically immune to Charizard's standard finisher
Which opponent was that? I really don't remember, unless you are refering to the Dusclops battle on BF?
Man, Ash's grass pokemon kinda get screwed over when you think about it.
Chespin - who should have always been Ash's - got worse. At least the others belonged to Ash and get to do some battling.
At least Mach Punch doesn't have to rely on hitting an opponent who isn't too heavy for it to work.
I really wonder why the staff just didn't give Sky Drop to Charizard. Really, we can basically turn it into Sismic Toss and it'd be the same thing.