>>54309916>>54301859The earliest work that I could consider a classic that I read back in the day which is still online and updating would be Pokemon Mystery Dungeon: Silver Resistance. It goes all the way back to 08, the early ears of PMD . As such, it has a lot of stuff from back in the day which quite a few authors followed from then on, whether they know it or not.
https://www.fanfiction.net/s/4298303/1/Pok%C3%A9mon-Mystery-Dungeon-Silver-ResistanceBesides that, there are precious few "classic" works out there which are still around, or of any mention. Mind you, most of the writers from that era aren't as "developed" as we have now in a sense, but there are goodies if you use the right search terms and search results.
The best of the very early era with all that entails from the fanon of the day would be this one.
https://www.fanfiction.net/s/2146127/1/Clouded-SkyIt isn't the only one, but after dredging through the muck, it's the best I have been able to find. I presume there are others that may be its equal, but either they are not on the site and lost when their web host went down, or they had been deleted and/or purged by their authors, moderators, or otherwise lost due to circumstance. A pity, seeing that there are hints of those stories floating about if you know where to looks.
Regarding more "recent" classics, quite a few of those have already been mentioned. But if I had to pick a few "sleepers" that would be of note, there are some that might be of interest.
https://forums.sufficientvelocity.com/threads/the-ninetales-and-her-breeder-a-pokemon-fanfiction.4819/Despite what the title may suggest, this does *not* involve breeding a ninetales. Rather, it focuses on a what if from the anime, namely being that Lokoko were to tag along on the journey with Brock and crew. From what it is worth, the prose is competent, the premise intrigue, and the execution is solid.