Imagine this context:
After we replaced the monitor, everything was fixed. Therefor the monitor was most likely the source of the problem.
I feel like given the right context, "most likely" doesn't sound strong enough, if that's the right word to use.
If I am 100% confident, I will say:
He is wrong.
If I am 70-90% confident:
He is most likely wrong.
But if I am 99% sure, what is a word or phrase that can get that message across?