I've only ever met classic Doctor Who stars, all of whom were excellent. All in one day, too - Tom Baker I had a photo with, he greeted me and put his arm round me. Peter Davison gave me a photo for free and was charming. Sylvester McCoy was a photo session and he was wonderful yet rather timid, surprisingly.
As for Colin Baker and Paul McGann, I had a photo session with Paul in which he was charming, but later on during the day, when everyone had cleared off, me and my friends ended up in Colin and Paul's booth. I discussed with Colin how I had grown up in the same town as him, and it was just a thoroughly amazing experience, relating local knowledge to one of my favourite actors who actually understood the nomenclature.
Paul went to the loo twice, one of my friends backed into him accidentally and he threw his head back and yelled "Bastards!", and laughed heartily. I then proceeded to surprise him by telling him, just for a few seconds, how much he means to me and what he's done for me. The smile on his face is something I'll never forget. He's my favourite Doctor and I love his audio stuff.
Katy Manning also gave me a free photograph and wrapped her arms around me to do so. The staff at the con said no free photographs, but Peter, Colin, and Katy didn't pay attention to this! Perhaps the best day of my life.
It was Collectormania MK Dons, and I'm going again this year. Constantly refreshing the site to see if any Trek stars turn up!