As people have said, either sentence is fine - I'd say that you have just dropped your key might sound a little passive-aggressive though, almost as though you're narrating somebody making a mistake.
The present perfect has a sense of a past event that is still having an effect on the present, so you have just dropped your key implies that a mistake was made and hasn't been corrected yet. So it might sound a little like you're saying "didn't you notice?", "that's going to cause a problem" or "are you going to pick that up?"
I feel like this is definitely softened if you say you've just dropped your key in a more casual way, or if you omit just (which can sound like you're immediately pointing out a mistake as soon as it happens). This is all really subtle, so I think the best advice is to avoid you have just dropped your key because that's very formal and direct, and sounds the most like criticism.