This isn't even a contender for within the first 10 actually. It is however among one of, if not the, oldest open thread on the minecraft forums due to the fact that I never 'rebooted' the thread like other mods have. Even the modloader thread has rebooted once, since originally it was CyborgDragon who wrote and updated it since Risugami didn't want to bother making a minecraftforums account, and when eventually he did, he kept the thread updated.
Also, ModLoader is also not the first mod ever made for minecraft. The first mod was made by someone (I can't remember the name) who made it, and then sort of left. As far as I know, it even predated the minecraft forums having a section for mods, and it was just passed around on IRC. Modloader is however one of the really early ones. This however is secondhand knowledge: It was a topic brought up on irc ~2 years ago and I'm going off of memory, since searching my logs would take a while.