I think it really depends on what style of quilt you usually make!
I tend to make dark colored quilts, so I bought a whole bolt of 108" wide solid navy blue fabric that serves me very well. I also buy wide natural-colored muslin in bulk when I find a good deal on the better quality kind, so between those two options I usually can make do with what I have on hand. I do still buy printed fabric for specific quilt backs sometimes too.
I'd also like to get a bolt of wide flannel, I am thinking a medium grey color would work pretty well for my quilting habits. But so far I haven't found a bolt in quality I like at a price I'd be willing to pay.