I suppose what you are getting at is story/acting vs. filmmaking.
Sometimes movies are so well made they win director, cinematography, sound, editing, set design etc. and cumulatively that is the best picture also.
Other times it isn't that so . much as it is the story and acting performances.
Sometimes it is both or all of that.
I've seen those three movies and two other noms and would have a hard time picking best picture among them for that reason.....but here's my brief reviews....
Once Upon a Time - great acting, story is a little meh until the end, and technically it has some great elements as usual for Tarantino, period piece aspect was well done.
1917 - technically off the charts, the size and scope is incredible, directing, cinematography, etc., the acting is good but not great.
Parasite - incredible story really, just keeps you mesmerized cuz it's so unique. some great acting too. directing, technical stuff is all very good also. really good to great in all aspects.
The Irishman - another great Scorsese flick with great performances. story is a little dry, period piece stuff is great. technical aspects did not stand out other than the de-aging of course.
Marriage Story - this is just full of great acting, and a good story but nothing special in terms of directing or technical things.
So I have not seen Ford v Ferrari, Joker, Little Women, or JoJo Rabbit. But plan to see 2 of those this weekend - Jojo and Little Women.
But based on what I've seen so far I think we might actually have the first foreign picture win Best Picture with Parasite. With none of the others standing out head and shoulders above I probably would give it to Parasite based on it being the most surprising thing I've seen this year in so many ways.
But I could see 1917 or any of those I saw winning actually. It's as wide open this year as ever I think. But that's also why I think Parasite might win as traditional voters will be split on those others and then the voting method they use becomes a factor:
They don't use a straight popular vote but something called Preferential Balloting. With more than 5 nominees the academy wanted to avoid giving best picture to some film that only got say 20% of the votes if it was well liked by just that small percentage but hated by others. So they brought back the old style preferential ballot method. In this voting method voters have to rank the nominated movies from 1 to 9. Then they disperse them effectively into piles with their top vote. If no film has more than 50% of the first place votes then they take the bottom movie out and redistribute it's votes to the 2nd picture on their list. Then again they see if one film has more than 50% of the votes cast. And they repeat that until some film does removing the bottom film and redistributing to their 2nd or 3rd choice if it was already redistributed, etc. etc.
So that's where I think Parasite can win. It might not be top among more than 50% but I think it's going to be split among 4-5 films and Parasite will be in the top 3 for many of them. That may be true of 1917 as well though. I think it's between those 2 personally.