In fact, for most of your sentences, the meaning is almost the same, and you could use any of them. But there some very small differences:
...who I was telling you about. (At some time in the past, I was telling you about him, until we were interrupted/changed the subject/went home...)
...who I told you about. (I told you about him in the past. This is very neutral.)
...who I have told you about. (You already know something about him.)
The last one is a little different:
...who I have been telling you about. (He is the subject of the conversation which we are having now. Or: I have told you many things about him repeatedly over a long period of time.)

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