I've been looking/was looking for you

[nelson13]

context: I look for a friend. He turns up.

What should I say?:

I've been looking/was looking for you.

[billmcd]

At the moment of arrival, as your context suggests, no difference and you would hear both (50/50) in AmE.

[nelson13]

Thank you very much.

[Raymott]

context: I look for a friend. He turns up.

What should I say?:

I've been looking/was looking for you.

In non-AmE, they have different meanings. To use the simple past tense, you'd need to be referring to a specific time.
"I've been looking for you" means "Up until now ..." To say, "I was looking for you" if you're still looking up until your friend's arrival, sounds quite wrong. (in AusE, NZE, BrE...)
"I was looking for you" could mean "Three hours ago, I was looking for you." (I wanted to buy you lunch, but I couldn't find you, so I stopped looking for you and had lunch by myself). To say, "I have been looking for you" in this context would be wrong.

That is, it follows the usual rules for the use of simple past and present perfect tenses.