I'm not sure is my answer. It just happened years ago. 2PM was not my first Kpop group. The two groups that I would consider my 'first' groups would be SS501 and SHINee, and 2PM came right after that. Taecyeon was not my first UB, but for some reason, I think he will remain my UB for a long time (he has been for years already).
Why is Taecyeon my bias of 2PM and my UB? Well, just look at 2PM as a whole. They are called the beastly idols for a reason. When did Taec take over my UB position? I'm not sure. I do know that I went back and forth between Taecyeon and Nichkhun. Of coarse SHINee's Minho and Onew were part of the mix too. For some unknown reason, my heart and mind tell me Taec is #1.
I think ultimately, his personality was the deciding factor.