I often have fun thinking about this one- of all the movie worlds in the Pixar films, which is the most ‘realistic’? I think it is in this order of plausibility, from most beleivable to least:
Up
Wall-E
Incredibles
Ratatouille
Nemo
Bugs Life
Toy Story 1-3
Monsters Inc
Cars
I think that has to be either Up or Wall-E, because Up has pretty much happened in the past, and Wall-E probably will happen. I mean, look at the planet.
True- both could happen with advances in technology- robots and giant spacecraft are possible. The voice collars and ‘dog-tech’ in Up are the only really implausible ideas- but the rest of it is firmly based in reality.
Niether movie violates the laws of physics or biology, whereas the rest do.
I would prefer that Toy Story and The Incredibles “come true”, but they’re both quite a bit of a stretch…
Toy Story more so than Incredibles.
Unless you were talking about elastigirl 
Ratatouille
Nemo
Toy Story
WALL-E
Monsters, Inc.
Bug’s Life
Up
Incredibles
Cars
You listed all of them oh that’s great…
I put it in order of believability. I mean Ratatouille and Nemo take place in real places, so, obviously, they would be one and two.
Oh! It was a rank system. Okay, I thought you were saying they were all realistic 
Problem with your ranking:
You envision a world in which rats and fish can speak as being more plausible than the world depicted in Up. Up could happen at a push (if someone invented the intelligent dog collars). Rats cannot, and never will, be able to speak (or cook!).
Thanks for playing by the way! Love it 
I would probably choose WALL-E over Up because that many balloons could never lift a house!
Here are my top 3
Up
WALL-E
Finding Nemo
The rats & fish don’t actually speak in English, but they still communicate with each other like they do in reality. Like in Ratatouille, Remy has a long speach in ‘rat’ to Emile, but then it cuts to him squeaking from the perspective of humans.
Wall-E
UP
A Bug’s Life
Finding Nemo
Ratatouille
Toy story
Monsters, Inc.
The Incredibles
Cars
Up - Any amount of balloon large enough is capable of leaving anything, and dog translation collars sound plausible in the near future if there advances in animal linguistics.
Wall-E - I don’t think Earth will reach a point where there is garbage everywhere (people will just move to the suburbs or to less developed regions). The singularity also seems a long way off.
Finding Nemo - The fish are speaking ‘fish speak’, and most of the actions obey the laws of physics. There’s just the question of intelligence and bond with offspring (what real-life clownfish will travel halfway across the Pacific to rescue his son?)
Ratatouille - Rats cooking are very implausible, but everything else makes sense.
The Incredibles - If humans mutated enough X-Men style, the world of the Incredibles is not impossible. Some stunt sequences like falls from great height and the RV landing on the highway without injuring the Parrs stretch suspension of disbelief a bit much, though.
A Bug’s Life - Again, the bugs could be talking ‘bug speak’. The only implausible things are the ‘bug cities’ and the use of natural items like leafs and fruits as ‘props’.
Toy Story - Very unlikely toys can run back and play dead in time everytime someone walks into a room. Although that would explain why sometimes things go missing or get misplaced…
Monsters Inc - A parallel dimension in the closet of every kid’s room? What happens if someone moves house? Or they realign their furniture? Not to mention how did something bought in a store can function as a portal, or how monsters scare kids in homes with no closets (like igloos or huts).
Cars - Totally out of the question, unless you believe the multiverse theory. The whole issue of the world being self-sustainable or self-contained (Do cars eat meat? How did they build those buildings? Who made the first car?) already strains credibility.
I don’t know. Probably Wall-E.
It would probably be possible to build advanced robots in the future, and there is a small possibility lots and lots of trash could cause the earth’s population to evacuate onto a large space station.
Don’t you guys realise the Monster World exists? I mean, seriously, it’s right there!
Haha, just kidding. I have to agree with the general consensus, Up is the most plausible Pixar world (which is sort of funny if you think about it- talking dogs and giant birds and houses lifted by balloons, and these are the most believable things!) and Cars has to be the least.
The incredibles actually can be believeable. Anybody watch that show called stan lee’s superhumans?
Yes!! I love that show
The Incredibles world is totally believable. 
The clothing, houses, cars, appliances all looked like they were from the 70’s which was awesome and realistic.