Pretty sure I win 
It's a fair attempt, but calling yourself "the last one" isn't enough to make you the last one. It was never going to be that easy.
You need dedication. Commitment. You need to spend every waking hour refreshing this thread, watching and waiting, dangling the prospect of victory before your opponents, only to snatch it away when they drop their guard, thereby gradually whittling away at their resolve.
And you need to hold onto hope when your own victories are inexorably crushed, again and again.
No, this thread isn't going to be won by word games and cheap tricks. It can be won only by blood.
Sounds like someone is salty they didn't think of it first
MY BLOOD IS LIQUID NATRIUM, MY BREATH HALOGEN. But let's leave my origin story out of this.
Calling yourself "The Last One" fails for three reasons.
The first reason is grammatical. The thread title is "Last one to post in thread wins", not "The last one posting in this thread wins". Infer a couple commas, tweak the capitalization, and you might read that as "The Last One, posting in this thread, wins", in which case the selected username could have some merit. But we don't have any reason to say that "The Last One" as a forum user refers to the same entity as the "last one to post in thread"...especially when "The Last One" currently
isn't the last one to post. They do have a valid standing as the last one to register for this forum...for now, anyway.
A better choice of username would have been "Last one to post in thread". Such a user would appear to win, simply by virtue of existing, whether they posted or not...but that brings us to the second reason:
Intent.
In a court of law, if Douglas Defendant sues Alice Plaintiff, we understand that, for legal purposes, Defendant is the plaintiff, and Plaintiff is the defendant. In the court, the intent of the law would supersede the apparent letter of the law. Any rights reserved to the defendant would clearly apply to A. Plaintiff, not D. Defendant. The proximate intent of this thread is clearly that the last person posting here should win, not that it be concluded by semantic arguments.
The third reason is trivially obvious.
