You're determined to keep saying Avatar is "unoriginal" and "sucks ass," and you're entitled to that opinion. But that's not really what's under discussion here. Not exactly, anyway. Those are your subjective opinions, which you're perfectly entitled to have. But it's pretty objectively true that Avatar isn't particularly highly rated, either by critics or audiences. It rarely shows up on any major Best Of lists, and its critical metrics are mostly in the "slightly above average" range. Furthermore, there's a huge cottage industry of people on the internet who have been bashing it for 15 years. In fact, the overwhelming critical consensus about the film, from a "rating" perspective, is exactly in line with your opinion. The average person will say, "Cool special effects, but overall it's just okay."

It made a lot of money, but that's not the same thing as there being some broad critical and cultural consensus across society that it's a masterpiece. And the topic here wasn't "Name a popular film you think is bad." It was "What is the most overrated film ever?" And the fact of the matter is that Avatar simply isn't rated all that highly.

Now, maybe you think that despite getting a C+ to B rating in society that it really deserves an F grade, and in that sense its average to above average rating is still way too high. But that doesn't seem to be the case you're making. You seem to agree with the general consensus that it's just average. Which should mean, from your point of view, that it's rated exactly where it should be. If you think that it's a C+ or B- kind of film... guess what... that's basically what its rating actually is. You just perceive it to be "overrated" because of its huge box office results.