Disclaimer: this is a ranking of words for "butt" - meaning the entirety of the butt, not individual sections. Therefore, "butthole" variations were not eligible - such as "anus", "crapper," etc.
"Posterior" honestly just sounds like the first word Hank Hill would use to describe his butt - basically, it's the most boring, old-timey word for butt that completely robs all of the fun of saying a word that means butt. It's like someone wanted to say "your behind" but thought that would be too crass.
"Arse" is just a messed up British way of saying "ass", but making it sound unnecessarily worse. And while "ass" can sound kind of nice, no one ever uses "arse" in a positive way.
Butts should not be described using sections of trains - some train-obsessed nerd probably coined this usage and now we're stuck with its existence forever. Thanks a lot, trains.
Listen, humans cannot be reduced to "sides." We're not CUBES or anything - we're PEOPLE. So saying "backside" and presuming I'm going to understand that means "my butt" is kinda nuts. Why wouldn't "backside" mean my actual back? Or the back of my legs? It's just a mess of a butt-word from all angles.
The term "trunk" implies some pretty big misconceptions about butts - it has the "caboose" dilemma, in that it's making the butt part of a car (stop comparing butts to sections of machines meant for transportation!), but also it implies you're supposed to leave things inside of it. And while you CAN put things inside the butt for pleasure purposes, you're definitely not supposed to LEAVE anything in there longterm.
Bottom? The bottom of someone is there feet, or at least their legs (if I'm being generous and including the feet as part of the legs). Using "bottom" to describe a butt is just confusing.
Using the actual anatomical name for butt cheeks - "gluteus maximus", or "glutes" - is just pretentious. You're trying to show off the fact that you know science terminology, or you're the kind of person who regularly describes their own muscles using the correct words. In either case, you're trying to imply you're better than me with your butt terms, and I won't stand for that.
This is definitely a term your mom used for a butt at some point (as in "get your heinie downstairs this instant!"), but it's definitely fun to say. If/when you ever have children of your own, you will DEFINITELY use it in the same way as your own mom did.
"Rear" (and/or "rear-end") is alright - but it has the same issue that "backside" has, where it's just saying "in the back." Not a very compelling word for butt, but at least it's short 'n sweet.
"Buttocks" is simply way too formal - the word "butt" is RIGHT THERE, but you're extending it for some reason? Why? "Butt" is perfectly acceptable and descriptive - you're just adding on an extra syllable and making yourself sound like Forrest Gump.
Very much in the "mom" school of butt-words, "keister" at least has a little extra flair to it by virtue of it sounding a little European.
The Yiddish word for butt - "tukhus" - is very, very fun to say, but is definitely a completely de-fanged word for butt. It just has no edge to it.
Yes, technically my butt IS behind me, but that shouldn't be the most distinctive thing about it.
"Cheeks" is kind of a cute term for butt - keeps the focus on the outer part of the butt instead of the butt as a whole. But that's also a problem - the main issue with "cheeks" is that it only implies the SURFACE of the butt, ignoring the meat of the butt and the butthole. With butts, you've got to consider everything.
I'll give "seat" this: it's the most philosophical term for butt. It's implying your buttcheeks form an organ seat - so whenever you sit down, you're never without a seat since you ARE always cushioned by your butt. However, this one loses points because it sounds like something my grandma might say.
Fanny is (yet another) mom term for butt, but also has the connotation of "the fanny pack," which is an extremely dad thing - but which is actually extremely useful when going to a theme park, so this wins a few extra points for that.
Badonkadonk is very fun to say....at first. The problem is, you can really only use it sparingly - it's way, way too many syllables to be your mainstay term for butt, especially when there are so many shorter and simpler words out there.
While it's use is limited to texts, the peach emoji is really perfect - it has an air of innocence about it (since it's technically just a picture of a fruit), but has the much more sexual implications since the emoji designers at Apple made it look SO BUTT-LIKE. They HAD to know what they were doing when creating this emoji - Jony Ive definitely spent at least 4 months refining this, no doubt.
Frankly, I've never made a dime off of my butt - but those who HAVE made money off of their butts traditionally have EXCEPTIONAL butts, so this moves up a few more slots than it normally would. If you can make money off of your butt alone, you deserve praise.
"Tush" is actually a slang term for "tukhus" - but it's a lot more fun, a little bit cheekier, and way less grandma-y.
I really don't have any good justification for liking "rump" so much - it kinda makes you think of crummy meat, but it's endearing.
Equating your buttcheeks to warm, fresh-out-the-oven dinner buns is the most appetizing way of talking about butts.
Well, we're down to the final five - starting with "booty." It's one of the more explicitly sexual terms for butt, it's fun to say, and you can make "pirate's booty" jokes. Really, it's just a solid butt word, beginning to end.
Maybe the CUTEST term for butt is "bum" - it's hard to think of it in a cutesy London accent, and London accents are undeniably the best accents.
Easily the FANCIEST word for butt is "derrière" - it's got an accent over the "e" and everything! But classing up your butt is such an incredible feat, it had to be in the top three.
The silver medal for butt words goes to..."butt." It's plain, it's simple, and it's to the point. It doesn't mess around - it's really the gold standard. You judge all other butt words against this one, because it's just so iconic.
And yet, there is one more word that beats out "butt" for the best butt word...
Of course - "ass." How could the winner be anything else? It can be extremely negative ("what an ass that guy is"), extremely positive ("what an ass that guy has!"), and everything in-between. But what it also has is the mystique of being a lot of kids' first "bad" swear word - and even though it's extremely commonplace in many peoples' vocabularies, there's a lot of fun and discovery to be had with "ass" that "butt" simply will never have.
"Ass" - congratulations, you're the best butt-word there is.