Well lets see,,, I have rifles with barrels that are 28", 29", 30" 32", 36", 37", and 42"...
As far as the most used,,, I would say it's the 36" barreled ones. They range from .40 to .54 caliber, and are anywhere from 7/8" up to 1 1/8" across the flats. They all have a purpose and serve the purpose they were built for just as the 42" barreled rifles do.
My all time favorite is probably my 37" x 1" - .50 caliber Douglas barreled rifle with peep sights. This is my AMS x-stick rifle that in a pinch I can shoot offhand if I want to. It has brought home many a medals for me.
Unfortunately, they have only been 3rd place medals but I'll take 'em considering the competition I go up against at State Shoot. It has also bought home many a monthly shoot bragging rights and 1st place certificates...
I like the looks of the 42" barrel on a longrifle and fact is, it's about the max length I personally can handle when I'm loading. I like to see the ball over my patch before short starting it. Any longer barrel and I'd need a step stool or have to change my loading technique... The way I load now is to ingrained in me to change.