>you can say "i win", "i won" or "i've won". <

The way I see it, it depends on your mental state of time:

I win - you're still living in the moment
I've won - still close to the moment, but coming out of it.
I won - it's all over and done with and you're moving on to the next thing.

The present perfect is about achievement or completion, no?

Then again, talking of mental states, it's the middle of the night and I don't guarantee mine!