I am asking because I often see brand names mentioned. The first branded wool I bought was a James C Brett which I've really enjoyed working with. I've also used Stylecraft but the jury is still out on that one. I've bought some Marriner wool because I found an offer but I've not knitted with it yet. I'd not heard of it, but apparently it's been around for a while. 
I've recently used some Sirdar, which so far I've been quite impressed with. I got some of the Bohemia super chunky, and I love it! It's so soft! And huge, so knits up quickly. 
So, what floats your boat/clicks your needles/has you hooked?