Since I roleplay on forums (even roleplay in general) very loosely, I can't name a favorite forum roleplay. But I can describe my favorite roleplay group that I was active on for about 2 years in-game.
In 2011, I was on a Warrior cat character named Silverheart, without a clan. I believe I was hanging out with some wolves (pretty rare right?...) and then I saw two clan's, Oakclan and Forestclan, fighting. I started to jump in their fight, trying to find out what was happening. After the argument was over, I believe I was invited to and joined Oakclan at first, but I was convinced to join Forestclan. So then I left Oakclan and joined Forestclan. I roleplayed regularly in that group, and it started to get big for me. Even a member took a photo with me in it and made a Facebook page. In 2012, I created a kit named Stormkit for two cats. Since the father was a mutant cyborg, the kit was part mutant. I remember spending time with my 3 other siblings, only saying "mew" for the first few times I roleplayed on that character. I also remember making an enemy named Blackkit. I grew (skipping apprentice-hood because my other siblings did) and I became a full warrior after awhile. A kit hated me then because she thought I would kill the leader to steal her place. I then got possessed by Tigerclaw's soul and killed one of the warriors (he wanted to be killed), and planning to kill the deputy so I can take the place (I soon got over that, don't worry lol!). Then probably in the beginning of 2013, the roleplay died after the original leader (who I forgot the name of) retired, once revived for a bit, then died completely. Stormfeather, with her mother dead and father and siblings lost, joined another clan, which I had never went on since late August. :B
The back-story before the clan was pretty legit. Forestclan branched from Oakclan; I believe the leader (was a warrior at that time) got angry at the leader of Oakclan and made her own clan.
Since this was so long ago, I don't remember much.
Even how horrible my roleplaying was (and still is), how I humiliated myself many times, Forestclan would always be my favorite and longest roleplay I have ever been in.