While I wouldn't select TDK as the best movie ever, I do disagree with people laughing at it.
I think choosing a "best film ever" is a very tough thing to do as films can be great, but, at the same time, unenjoyable (I mean, who really "enjoys" Schindler's List? It's a powerful, powerful film, but it's supposed to be very sad and unsettling).
They can be great, but quite unmemorable (I mean, half of the movies that win best people, everyone forgets about within a year...but they're still very good films).
What TDK, I'm sure, does for some, is it provides an extremely enjoyable movie watching experience, while enforcing that enjoyability, with quality film making that (as most will admit) is one of the better films of the year.
I think that combination of being both a good (or great) film, and being highly enjoyable may very well propel it to the top for some people, I don't judge them at all for that.
As great as The Godfather is, I've seen it once, and really doubt I'll see it more than two or three more times in my life, whereas movies like TDK, or a Star Wars movie, etc., I'll watch again, and again. And it can be argued, that that's an important feature for a movie to have; enjoyability, "rewatchability", and memorability.
I also disagree that people think TDK's great just because it's the newest Batman movie. That's ridiculous. Will that make people overrate the film when compared to other Batman films? Sure, but unless that Batman remake 20 years from now is genuinely better than TDK, I highly doubt it will be viewed as such.
And TDK is a very memorable, enjoyable film. I haven't seen a single movie that's made almost a billion dollars worldwide ever be forgotten. Whether it be Gone With the Wind, Star Wars, or TDK. It's a memorable film. It's a very good film. Is it the best film ever? No, but that's my opinion. And I respect the opinions of the people who feel different.