The most accurate .50 BMG rifle is the bolt-action rifle made by McMillan Bros.
I believe that that particular rifle has set two world records; one being civilian for long-distance group (I believe it was 5 rounds at 1000 yards), and the other record that I believe was made with this rifle was the roughly 2,350 meter (yes, two thousand three hundred fifty meters, or about 1 and 1/2 miles!) confirmed kill made by a Canadian sniper team in Afghanistan.
Now, keep in mind that you are going to pay for that accuracy. I know that the McMillan bolt-action .50 is at least $5,000, probably more.
My FFL has one (along with several other .50 BMG's) and I think he told me the price a while ago, but I forget for certain.
All that being said, I think that most of the major brand .50's on the market are more accurate than most of the people who will ever shoot them.
I have a Serbu .50 and it has recieved many excellent reviews for accuracy. It also happens to be one of the least expensive BMG's, at around $2300 or so.
Do NOT confuse "inexpensive" with "cheap" when talking about the Serbu; it is top-quality all the way.
What price range are you looking at?
What are you looking to do with this rifle?
If you are serious about long-range competitions, you'll probably want to go with something like the McMillan.