In echoing some of the other posters, it will all depend on several things:
What is "long range" to you? If 200 yard benchrest is your idea of distance, it will be a dramatically different set of viable options than, say the guy in Idaho, that has 1500 yards available 20 minutes from his home. You first need to figure out how far you realistically are wanting and able to shoot on a consistent basis.
What is your budget? Are you looking to grab a Ruger Precision Rifle with Vortex glass (awesome combo by the way) or do you want to dip your toe in to the custom world? Both guns do roughly the same thing, the expel a cartridge out of a barrel, the custom has more style and operates more smoothly.
How do you feel about recoil? If you're planning to run a brake of some type, recoil can be mitigated to a large degree. I'm not much of a brake guy and don't love recoil so I tend to look at cartridges that will perform to the distances I need with minimal recoil. If you're not averse to it, then the world is your oyster.
Do you have glass? If so, that's awesome. If not just know that it's almost impossible to spend too much on glass.
Do you/will you reload? This is a big one as without it, you will never be able to realize the full potential of any cartridge. Factory loadings are really solid. Federal Gold Medal Match will definitely shoot well from a variety of rifles typically. A few years ago a guy won the NJ State FTR championship using factory ammo (He got lucky and it shot really well from his rifle. He now reloads and shoots even better). However learning to reload, even for just the one cartridge, will teach you more about your rifle than many other things. And, you can truly optimize the load to do what you want with the rifle in any situation.
Take a look at Tibosaurus Rex on YouTube. He has a series that will take you deeper in to long range shooting than you can imagine.
FWIW, if you look at any of the PRS guys, they are typically shooting the latest and greatest. Many of the cartridges are almost exclusively for handloaders, however what you NEED to be looking at is the bullets being used. Most of them are shooting 6.5's (although less and less), 6's and 22's. The new 22 offerings are amazing and will shoot inside the 6.5's at anything less than 800 yards with almost equal wind and drop corrections. That said, the 6's are an amazing bunch of cartridges.
Currently I shoot a:
223Remington. I was shooting it at 600 in matches but the drive got to me so I will be simply shooting it and having fun.
6mmBR Norma. An amazing cartridge and really fun to shoot. In better hands than mine a mildly improved version of this cartridge shot 5@ 1.1" at 1000 yards .
22BR Norma/Lapua. I will be building this this summer. It's a necked down 6BR shooting 22's faster/farther.
Good luck in your search. Drop me a PM if you have any questions that you think I can help with.