I am a serious varmint hunter, and I use a custom rifle. The cartridge I use is the 300 WSM which is designed around .337 tightneck match chamber, Nesika Model K Action, Jewel trigger, Bartlien 30", 1-15 twist barrel which pushes the 125grn BT at 4000fps. I use this rifle for shots from 100 to 800 yards.
I really think the 300 WSM is a very versitile long range cartidge. With a 30" barrel, it will push a 210grn bullet at 3000fps. Many 1000 yard bench rest shooters are turning to the 300 WSM because it is performng very well at these matches. It has a ton of power and will allow you to kill any type of animal in the U.S.
I think there are a few cartiridges that would fit the bill as the best overall long range cartridge. Here are a few I would mention; 6.5x284, 7mmRem Mag, 7RUM, 300 WM, 300 WSM, 300RUM, 30-378 Weatherby Magnum, 338 Lapua Magnum and the 338-378 Weatherby Magnum.
There are even better overall long range cartridges you can use. They are the 270 Allen Mag, 7mm Allen Mag and the 338 Allen Mag. These are the ultimate long range cartridges you can own. However, you have to have a custom rifle to shoot them and the person you need to contact to have one built would be Kirby Allen (Fifty). He is the designer of the above mentioned Allen Mags. You will not find a more powerfull, flat shooting line of cartridges anywhere. Send a post to Fifty (Kirby Allen), and he will give you a much better description about his line of Allen Mags than anyone on this site can.
If your thinking about having the ultimate long range hunting rifle built and you want the ultimate cartridge, have Kirby build and chamber your rifle.