Hi Art:
I think this horse is starting to stink a little, so I'll try to throw some dirt on it and try to answer your question.
I think Lou and I essentually agree. Your initial question was, "What is the most important ... component of a benchrest rifle ???" To me the most important "component" is the scope. I assumed, from your question, that you were contemplating a tried and true caliber and so, I answered the "scope" is the most important because a scope that won't hold zero will have you chasing your tail trying to figure out what's wrong with the cartridge, barrel, bullets, powder, bedding, etc.
Lou didn't bipass my assumption and said the cartridge is the most important thing, because if you don't have a cartridge that's capable of producing exceptional groups, the scope, barrel, bullets, powder, yada yada become irrelevant.
I think, although I'm not trying to put words in Lou's mouth, that if you base your rifle on tried and true accepted BR accuracy round, such as the 6 PPC or the .22 SPPC, the most important component is the scope, followed closely by the other components mentioned in your thread.
When your scope holds zero, its amazing: (1) how well your rifle shoots and (2) how quickly you can diagnose other problems. When it shifts around and causes you to "throw" shots, it can be the most difficult thing to disgnose. Bad barrels usually let you know they are bad -- they foul badly and are difficult to clean. Bad bedding, its one of those "I know it when I see it deals." Generally though, the gun just won't group at all and there are none of those tell tale signs that the barrel is the culprit. Bad bullets tend to tease you with three and twos or four and ones (Bad barrels can do this two, but barrels tend to follow more of a pattern whereas the fliers from bad bullets are more random). But bad scopes, unless they are REALLY bad (such as when the reticle falls out or is canted with respect to the turrets, or you shoot huge diagonal lines in really mild conditions), are difficult to diagnose and are always cheating you out of some part of your agg.
I hope this answers your question. If not, reread the other posts in your thread and just accept that no one is wrong. All the things mentioned have to be working to have an accurate BR rifle. If anything is missing, you are less likely to be a winner and more likely to be an also ran.