Fantastic Mr Fox might be Wes Anderson's most Wes Anderson-y movie. It's got all of his bag of tricks featured on full display, such as:

• Symmetrical framing
• Direct writing with a monotone delivery
• Stop Motion animation
• Panning shots of a dollhouse like cross-section of a building with all our characters in different rooms
• Excessive usage of 60's-70's music, including 2 uses of The Beach Boys
• Jason Swartzman acting like a little cussing brat 
• Vintage technology 
• A scene with a close up of a letter which is being read aloud
• Alexandre Desplat
• Various members of the Wes Anderson Gang™️ showing up for legit 1 or 2 scenes just to renew their Wes Anderson Gang™️ membership cards

And finally, and perhaps most importantly:

•An existential undertone where the main protagonist who is eccentric and driven ultimately realises that this is a facade built up by them to hide some kind of deep, underlying fear or flaw which they learn to overcome and get over

And that's what I think makes this movie great. It's Wes Anderson's most mature and most childlike film. It's simultaneously dealing with a midlife crisis and a coming of age story all at once that's able to break through the excessive, fantastical style and able to deliver real emotional sincerity, which he's able to do in his other movies really well to (except Rushmore, Ash's character arc here is far more developed and nuanced than the little cussing cusshole Schwartzman played in that film) but Fantastic Mr Fox is able to hit slighter harder in this aspect.

Mr Fox's desire is to feel sophisticated and cunning, he wants to be in a place of high class and he's determined to aim higher and higher, so it's all about him learning to be content with being with the people he loves and making the most of a dire circumstance.

It's not my personal favourite Wes Anderson film, but it's certainly in the top-tier of Wes Anderson. It's so much fun, it's endlessly creative and endearing, and it's emotionally resonant and moving as a piece of art.
And it's certainly a lot better than that piece of cuss that is Rushmore.