Hmmm, I wouldn't say there's any hard rules per se, but some traits do help.

Appearance-wise I like characters who lean towards nerdy or dorky. Glasses are a big plus as is curly loose hair. Usually with a small/thin complexion. Freckles are a nice bonus, too.

Personality wise, I tend to lean towards characters who are reserved and maybe even a bit shy. Characters who are caring and kind deep down even if it doesn't always show.

Overall, the biggest factor probably is the role they play in their series and how their character arc develops. What works and what doesn't is very story dependent so it's hard to pinpoint a specific detail, but at some point during the series I'll find myself rooting for a particular character and I'll know I found them.

From the mother series? I like Tony x Jeff and Ninten x Lloyd a good bit.

From other franchises, I'm quite fond of Taichi x Koushiro from digimon. Ken x Daisuke is fun too, though I'm slightly less invested. Bojji x Kage is kind of a weird one that I got into thanks to an artist that used to draw them a lot. Yuzuhiko x Fujino from atashin'chi is a rare pair I wish got more attention. Things like maxvid when I'm feeling freaky. TBH if two characters make a cute gay couple I will ship them to some extent.

Naturally I ship all the faggot stuff too. Creek, Luberto, Pico x Boyfriend... Y'know, all the cringe stuff the gays are into.

I also ship a few of my friend's OCs, but those are probably less relevant.

For me it wasn't really a single moment where it all became clear. Since I was in school I never really felt particularly attracted to girls, but I figured I was too young and that was just something that'd happen later. Same with TV characters, where I would often pass up the girls when they showed up like, in bikinis and stuff and would pay attention to the boys instead.

These were all things that happened subconsciously for the longest time without me really trying to think what they could mean. I come from a fairly religious family as well, so the idea of being gay simply was unthinkable. It wasn't something I was programmed to ever consider.

Then I started frequenting fandom forums online where people shared stories/headcanons, and I really found myself gravitating towards the yaoi stuff. The idea just felt nice. The stories felt compelling. I started to fantasize about them. You wouldn't believe the mental gymnastics I had to make to convince myself I was straight, but I did.

It wasn't until I made some good gay friends online that I actually put the pieces together and realized that many of the things I was going through, they were going through as well. Everything just made so much sense.

To an extent I kinda regret that it wasn't something I had the opportunity or tools to express or understand while younger. It was a long journey but at the end of the day I'm glad I made it there.

Hmmm, to be completely honest I've not been the most prolific fanfic reader in recent years. There's a few I remember fondly, but I've be hard pressed to find them at a moment's notice.

I'd hate to leave it at such an unsatisfying answer though, so I will offer this fanfic from a few years back that I think exemplifies what I tend to like.

https://archiveofourown.org/works/24228628

Might feel a bit hard to follow if you aren't familiar with Digimon, but I enjoy the doting, tender gay longing. The pulling on heart strings. The expectation. Did I mention the gay longing?

(Maybe I just enjoy stories that make me agonize internally?)

It sadly was never fully finished, but it's still a good heartwarming read.

If that's too much cheesy fluff for you (I understand), I'll also throw in this one that's a little more silly (and lewd) in nature, with a twist ending that still gets a chuckle out of me:

https://archiveofourown.org/works/17713235

Dunno if anyone would call these the BEST, but I like them.