I didn't much care for Avatar when I watched it and I love both Endgame and the MCU in general so obviously I very much want to see Endgame on top. That's not to say I don't recognize Cameron's achievement though. I don't consider success at the box office random or undeserved. Any movie that manages to draw in big crowds to the theaters is worthy of respect and that fact alone means that these movies did something right that the audience liked. So if Avatar does lose its crown to Endgame it will still be worthy of a lot of respect and admiration.
Endgame though... I won't lie, there is an emotional element to my desire to see it prevail. Not that much for the movie itself or the MCU, they're both so wildly successful that a few more millions make very little difference in the grand scheme of things. But for me and I imagine for every person out there that grew up reading comics as a kid, it really feels special to see the characters and stories we grew up with becoming part of the cultural mainstream and embraced by people all over the world. It's really rather awesome.