I've decided to give my best friend an ultimatum
What is the ultimatum going to be about exactly?
It's an ultimatum that will answer if they stay my best friend or not.
If you decide to go through with this and give your best friend an ultimatum then they may stop being your friend altogether, because no one wants to put up with that sort of thing. You should just let them know if something is bothering you and the two of you could work through it.
Nah, I don't want to talk about it. I'm just going to give them a time period to do this for me.
Giving your friends tests through ultimatums ultimately says that you don't trust them and that you may turn on them when things don't suit your purposes. Think hard about the negative messages you are sending here.