I use a local guy who doesn't even has his MoT testing facilities. (He takes vehicles to a garage that he has an arrangement with). I'd regard a good independent garage as far more reliable than any national firm.
However the one national firm that has given me excellent service in the past is KwikFit. I took a car into a regional firm because the brakes on my car were soft and, even after they'd 'fixed' the problem, I couldn't tell much difference. (I gave them a second chance and they still couldn't detect any problem). So I went to a big national firm and they also said there was no problem. So I tried KwikFit, who quickly found two leaking brake cylinders. I used them several times after that and always got good service. (However I suspect that, like all of the big chains, the quality of service depends upon how good the manager is. I've also heard plenty of moans about other KwikFit branches!)