I've never heard of Mendip Rail to be honest....
Mendip Rail doesn't actually haul their own freight from the Mendips - DB Cargo is contracted by Agrigate Industries to haul the stone traffic from Somerset etc.

To be fair this is posted on a Rail enthusiasts forum I don't think he was asking for our views as a freight customer, and I think one or two of you are being a little bit mischievous
As ScotTrains has mentioned we do perhaps have some views on this but maybe so long as we see a freight train we are happy whatever toc it is.
I am rather fond of GBRf but perhaps because they are not quite as common where I am and I like what they do with some of their locos such as 'Evening Star'
I've voted for GBRf, on the basis that
1) as I understand it, they arranged from the start a contract under which their drivers work more flexibly than those of other freight companies, which has probably been a contributory factor to their success;
2) their MD John Smith appears to have a very solid grip on affairs and an ability to come up with arrangements that win business for the company.
so we are choosing on who paints their locomotives a nicer colour? Some criteria for judging would be helpful.

I do love Colas Rail, I must admit it was tough. But Colas operate the NMT for Network Rail and also have those super grids and 70's! Not to mention the Colnbrook to Lindsey 60 ... :P