Well I'm pretty new to this fandom, and I haven't played all the games or watched playthroughs of all of them yet. But I started to like Roxas/Namine after CoM and KH2... I know they don't have a ton of interaction, but they are my favorite male and female characters respectively... at least at this point

but that's a superficial reason to like them together XD More concretely, during their short interactions together, I just felt like they resonated well with each other. They both have a sadness to their characters and they share the questioning of their identity, their autonomy. And they open up to each other about those fundamental questions. I know that most Nobodies are likely to struggle with those questions at some point, but you see Roxas and Namine being the first two in the series (I think?) to really be puzzled and bothered about who they are, questioning their purpose in the world. So that really struck a chord with me and I thought, well they would be good for each other because they can understand what the other has gone through, and support each other.
I dunno, I can just imagine them being high school sweethearts, growing up in Twilight Town together. They would be the skater boy/artsy girl couple who are kinda misfits but they don't care because they have each other. Wouldn't that be sweet?? Seriously, my heart is melting
As for Sora/Kairi vs. Sora/Riku, I have an opinion on this too.... while playing KH 1, I thought that Sora and Kairi had some very cute moments. Like I teared up at Kairi's, "Don't ever forget. Wherever you go, I'm always with you." (Who didn't though XD) I also think it's really cute how Sora thinks of Kairi as his light, and she gives him strength to overcome hardships. It's the sweetest T_T however, I actually also got the impression that Riku was a love interest for Sora in KH 1. It just felt like, as the game progressed, Riku was trying to save Kairi but he was also trying really hard to get Sora's attention. So I was like, "Hm. Maybe he likes Sora?" And then at the end of the game when they get separated and there's that whole montage of when they were younger, back on the islands, I was like, "Wow, these two had a bromance." LOL.
Looking at more recent games, especially 3D, I got the distinct feeling that Riku is in love with Sora? Not sure if the writers intended to write him that way, but there were so many moments where he would just think of Sora, kind of like how Sora thinks of Kairi, as 'his light.' It seems to me like Sora/Kairi is requited while Sora/Riku is one-sided. It's kinda sad though, because I really liked Kairi's character in KH 1. She was so spunky and cheeky while also being adorable. But as far as I can tell, writers dropped the ball on her characterization, starting as early as KH 2. On the other hand, Riku has gotten some very consistent development, not just his character but also his relationship with Sora. At the end of the day, I don't even know which one I prefer

probably Sora/Riku because Kairi feels so flat now compared to KH 1. But that will never be canon. You can never truly win
OMG this is so long o__O I have a lot of thoughts about shipping I guess XD