I honestly think you're thinking of this film in the wrong context...

It's not a comedy...
It's a surrealistic crime caper with comedic elements, mostly because of their irony.

Many say this is the "funniest movie ever."

Many of those people are also trendy. There's always going to be those people. They hype it up as something it's not...and they probably also forget some of the lulls in the laughs while the plot advances.

I'd say it's a pretty funny movie, but not the best Coen brothers movie. It's got cult status, and the interactions between the Dude, Walter, and Donnie are really the crux of the humor. If you don't like the three of them and how they work together (or don't) then you/re probably not going to find much of the movie very funny at all.

The film is very dark...that is, it's a dark comedy. Dark comedies make us laugh at things we wouldn't normally. And those kind of jokes also have a way of not landing, depending on your sensibilities.

At the OP...could you imagine having watched this movie in the theater when it first came out...with no hype? Do you think your impression might be different? Many of the readers here at VGC are old enough to have been there...so it does have a special place in our hearts for being just a little different than all the other crap they release every year.

I don't think it's a masterpiece. I think it does what it's supposed to do effectively, which is show you the attitude of the dude and indulge subtle and dark humor. But I don't think it's a masterpiece for purposefully having a sloppy structure presenting a lazy attitude, meandering dialogue because it supposed to be that way, and subtle humor.

I don't think it's clever or great, it just does what it does and that's what it's supposed to do. The actors are good.
In fact, the way it embodies that attitude actually leads me to feel that way about it. Not worth criticizing, but nothing special that I'll think about much.

I would probably have to watch it a second time, this time without the anticipation for the next scene to make me laugh. Based on my memory though, I predict that I would find it average again.

I suppose my example of a funny movie would be one where I could talk to friends and say "Remember that moment where character A did Z? That was hilarious." I sadly cannot think of any moments in TBL that I could use as such an example.

I've only watched it once, thought it was ok. I never had the desire to watch it again, though after reading some peoples takes on it, I may have to give it another go...

Yeah, well, that's just, like, your opinion, man.

Such a great movie. The relationship with The Dude Walter and Donnie was the best to me. The other characters adding problems and being quirky was just amazing

You know how some games are really cool when they first come out and 10 years later they age badly?

Same principle applies with movies. At them time it was considered good but now with what else has come out people's views change.

The movie had nihilists, what more do you want?

Then there are movies/games that go the other way.  Not appreciated at the time, but later are considered classics.  The Big Lebowski is clearly in this category.

I think what the OP is really trying to say is that he doesn't understand why his sense of humour differs from a lot of other people's.

Why do some people find farting gross, and others find it hilarious? Why do I think peas taste like crap, but other people love them?

Having a debate in regards to personal taste is beyond pointless.