The Kiron 105mm f2.8 is a super lens, and it's the one that really got me hooked on concert photography, but I have to honest:
The Sigma 28-70mm f2.8 EX Aspherical has barely come off my camera since I got it. That's gotta be my favourite.
It's so versatile, it delivers good results when properly used, and it feels ideal in terms of size for me. It has become my main lens when reporting events.
It also has this weird "purple cross"-effect with bright spotlights when stopped down, but I actually like it: