Borg Boy, hands down. I've written up the experience before. I'll dig it up and edit this reply to quote it.
Also, Jennifer Lien gave me a bad impression, though I've been told it was probably just shyness/nerves on her part. It just seemed very rude to me. The first time I saw her at a convention, as I was one or two from the head of the line at her autograph table, I heard her ask the staffer sitting beside her, "Do I have to talk to these people?"
ETA: Here's my account
of my encounter with Borg Boy.
Goodness, that's quite an experience with Manu. He was out here last year with Jeri Ryan, and I found him quite friendly and entertaining. Perhaps he grew up a lot over the years or perhaps you caught him on one hell of a bad day.
I've had mostly good experiences. Wang and Dorn were quite fun, Ryan was charming (though damned expensive), Sirtis was friendly if a bit prone to swearing, de Lancie was a little cold but didn't charge for photos (and I was, shamefully, inadvertently rude to him - he would have been entitled to have a go at me, but didn't).
Kim: I'm detecting some weird technobabble.
Janeway: A possibly dangerous anomaly that we know nothing about?
Kim: Yeah. I suppose we should steer clear.
Janeway: Ha! Good one, Ensign. Take us in, Mr. Paris.