When I was at the gym almost everyone used 12 lb body bars (the blue ones) for everything.
I really depends on your strength, and what you want to use the bodybar for. There are several bodybar tapes that use the bar as a sort of pilates add on for core work. I use a very light bar for that kind of stuff.
On the other hand, I find even my 18 lb bodybar next to useless as a weight add-on for lowerbody traditional strength training, i.e., lunges, squats, etc.
My final point, for added confusion
, is that I can't use more than 12 lbs for alot of shoulder and tricep work.
I have a 4, 10, 12, 15 and 18. I use them all! (sigh).