Yellow. Even though just about nobody else does that. I'm just obsessed with having a following buddy, I guess, and the disassembly is awesome. It's sooo painless compared to endlessly searching for offsets, fiddling with indexing your colors and using tools to compress and insert, repointing all the time, and rooting through uncommented code like in Gen3, and it's Followme is better than any generations' if you're looking for just a couple of pokemon to follow you because you can personalize those emotion sprites so much.
I did hack Firered a few times, and got a working across maps all the way to Johto follow-me going, but the ledge bug was severely annoying (even though it'd just respawn eventually). I have no idea if that's been fixed yet, but, maybe I'll finish my Fire Yellow project one-day anyway, or even post the incomplete version, even if it's kinda sucky, simply because I'm tired of people incorrectly saying you can't have Pikachu follow you around on multiple maps and that way they can see and just steal the way I did it (although I did say my method years back; I've improved my surf-disappears-pikachu method since then though - just directly editing the surf script to disappear sprite FE works well). It's just a pain to implement, is all. But I now think I might enjoy new pokemon patches that's come out since way when, so, that would require starting over. :/ And it would be nice to have bugfixes, or use the completed follow when it comes out (although at that point other people would surely make a yellow remake so I'd have no need to). But redoing a whole project isn't fun, so, yeah, probably not.
Maybe I will hack Firered again, especially if it gets a disassembly or I'm do something that will involve lots of space or mapping. Biggest downside to Yellow right now imo is the space, and mapping is a pain. I won't hack any of the other of gen 3 or Gen 1 because they don't have Followme, and I'm kinda obsessed, lol. I do admit to having a LOT of nostalgia for gen 2, but I can add gen 2 pokemon to Yellow sooo the only big loss is the phone, two regions, and day/night.
I miiight make a mild exception for Emerald sometime, just because I faintly recall you can team-up in that, and I love the idea of basing a hack around having a battle-buddy or being part of a team where you are given a partner; a Jessie to your James, or James to your Jessie (although Team Rocket is a little overrated :P). I've never seen a hack like that. Although it might get annoying not being able to control half your team, hah. So I guess I should say I have more of a buddy obsession than follow-me obsession. I'm a complete sucker for that.