d'you think the obsession with the most smiley ones is psychological? the constant pursuit of what he can never have...
Since we have no input from Russ himself on this, it's all speculation. Here are some possible reasons he likes toothy girls that have been discussed previously in the thread, though I don't believe they've been summarized.
1. Like you said, the pursuit of something he can never have. He might see it as validating to land a girl with a big smile to make up for the one he can't perform. To be fair, even if Russ' mouth did function properly, he wouldn't be able to truly smile anyway, as a true smile requires happiness. He would just be faking it like everything else. But he may see landing a toothy girl as making up for his own lack of smile.
2. It could be that this is one of the things he believes society thinks is very attractive. Its speculated that his ideas of attraction are derived from what he thinks other people find appealing (edit: to a much greater degree than the average person), and if he can obtain a girl with those features then men will be envious and want to be him, while other women will be see him as valuable, become jealous, and want to sleep with him. It could be that in his mind a big smile represents the top quality female seen in 80s and 90s romcoms that he bases many of his perceptions from.
3. This is a bit more out there, but that toothy girls remind him of his sister, who seems to have the big toothy smile from the limited pictures we've seen of her. A central part of Russ psychology is that the revelation of his birth parents keeping his twin while putting him for adoption had a major impact on his self-esteem. This likely also played a big role in his views toward women, as being "the face not even a mother could love" likely made him very insecure, and hateful towards his mother and jealous of his sister. She also lives in LA and works in "The Biz" and is therefore an opposite gender, successful version of Russ. In his mind, him bagging a toothy, successful, celebrity woman would be the ultimate "fuck you" to his birth family. There's also speculation that due to Russ viewing sex as having power or control over women that he may want to fuck his sister as some sort of twisted revenge fantasy. I think this is a bit too speculative to make claims for currently, but who knows when dealing with people as unhinged as Russ.
4. Adding what
@Pip Squeak mentioned on the following page, as I think it's also a good theory, and want to group these ideas together in a single post.
"Russ is a narcissist, nothing is more attractive in his mind than his own fucking face. His mouth is, for obvious reasons, extremely noticeable. His rat teeth hang there in the middle of his dead features and he seems to think he has nice teeth, he whitens them pretty often from what I can tell. He thinks these women reflect his ‘real’ face, how he’d appear if he didn’t have moebius. They’re just the female version of himself. He truly thinks he’s that definitely not photoshopped picture he uses but that somehow, he’s a solid 9 in it."
It also could be a mixture of some of these things, or something else entirely that I have missed here.