As with any fishing gear, it largely depends on what the local shops stock. I will occasionally take a blind punt (recent examples being DH line and some Browning hooks) and buy online, but I prefer to see even the smallest item in the flesh. Drennan invariably score highly due to their wall to wall availability. Every last one of my local shops stock Drennan gear, even if only the terminal tackle. Preston aren't far behind in terms of availability in a large number of tackle shops.
I have my favourites but that's largely down to long term use and faith in a particular brand. When I was doing a bit of carping and barbelling, the majority of my terminal tackle was either Gardner or Nash. For some reason, ESP never featured strongly, despite better availability.
For match/general coarse fishing, it's a bit of all sorts. I'd guess somewhere above 50% being Drennan. Add a dash of Dave Harrell, Preston, Browning, Cralusso, Guru, and Greys and I'm largely covered. Fox Match used to get a bit of my cash. Since they became Matrix, I barely see their products. I'm sure their stuff will be just as good as it was under the old brand name.
Even with products that I tend to favour one brand over others, I'd not claim that brand to be universally better than all others. I'd also hate to be entirely restricted to a single brand of any item. Fixed spool reels are probably as close as I get to single brand buying. However, I do have (or have had) other brands and I'm always keen to try any new stuff, regardless of brand, if I get the opportunity. I'm not remotely faithful to any brand when it comes to rods.
If forced to pick one brand for terminal tackle, it would be Drennan, though I'd not be happy about it. If forced to pick one brand for fixed spool reels, it would be Shimano. I'd still not be too happy to do so. If forced to pick one brand of rods, I'd probably end up having some sort of existential breakdown.