+1 on Colin, who is based near Pewsey in Wiltshire.
Colin mounted did this roe buck for me some years ago. These photos were taken in the last 30 minutes, and you can see that it's just as fresh today as when it was first completed:
I've included a close up of the eye, as this seems to be one of the areas that degrades as taxidermy ages.
Colin's website can be found here:
Colin Dunton Ltd : Taxidermists for hunters, collectors and museums
I would strongly recommend having a look at several different taxidermists and finding one whose work you like for the specific species of animal that you want mounted. I don't believe that there's any such thing as the "best" taxidermist, since not only is "best" a subjective measure but much of the beauty with taxidermy is in the eye of the beholder. Some want straight forward shoulder mounts, others might look for a more animated pose. Only you know what appeals to you.
For example for the muntjac I had mounted I went to Dave Hornbrook in Cleveland, as I like the way he does muntjac. This is a photo that is on his website from when the buck was first mounted:
http://www.taxidermists.co.uk/img/dht_110404/muntjac_head.jpg
And here are some taken earlier today:
As you can see with the eye, whilst there is a fair amount of dust there has been no degradation in the taxidermy. This buck was shot at probably just about the worst time from a taxidermy perspective - as it was moulting into summer coat - but Dave did a great job preserving as much of the coat as possible.
Where I do agree is that it is worth paying for good taxidermy. If you're going to spend money on getting an animal mounted then you're going to have it on the wall for some time. Better to go with a taxidermist whose work will last than to save a few bob and then find it looking ragged and moth-eaten in four or five years time.
You're welcome to see either of the above mounts "in the flesh" if you happen to passing the Swindon/Oxford area.