My first BJ was given to me by a 14 year old girl . I should mention that I was also 14 at the time. The quality of the BJ was really not very good. In her defence, she was drunk and it was her first time trying. Needless to say, any BJ is great, simply because your dick is in their mouth and that is cool. The first one is certainly an invigorating and exciting experience. However, since that time I have had many BJs that I have enjoyed much more.
My first backy was on a homemade kicker in my backyard. It was alright, but wasn't nearly as rewarding as my first backflip in a terrain park , with friends and people on the chairlift watching, and the rest of the run to ride away in style. Yes, this was a great feeling....
In conclusion, both my first backy and BJ were actually a little disappointing. However they were exciting at the time, and led to much bigger and better things. If I'm forced to choose, I would say the BJ was better because dick in mouth.