When the artists are having a good time, I'm having a good time.
The keyboardist for After Forever gets my vote. He was always smiling and nodding his head in a 'THIS IS SOOO AWESOME!!!!!!!' way. I loved it.
My vote goes to Pagan's Mind bassist Steinar Krokmo. As someone in another post pointed out, he looked like he'd just won the lottery.
If I were to make up a band using only the members who had the happiest time on stage imo:
Joost from AF (Keys)
Steinar from PM and Christer from FK (Bass)
Damian from Threshold (Vox)
Bill from Cellador and Mattias from FK (Guitar)
Bjorn from FK (Drums)
So you'd take FK and add some more members to them
A twin-bass attack could be interesting. They should do this as a side project!!
Christer from Freak Kitchen, when he was singing AC~DC. The microphone stand backflip was so awesome. He was truly, truly content with the entire world for those two songs. AND WE LAUGHED OUR ASSES OFF! That guy was born to be a performer.