first, does any gun manufacturer offer a 26" barrel on a gun, or is 24 the max for factory? also what would be the best all around caliber out to 500 yds.
AJ is right, there's a lot of questions that need to be asked and answered before somebody can really give you a 'best'....
But hey, I'm an adventurous guy, so I'll just hang my opinion out in the wind and see what you think....
Remington 700 SPS Varmint in .308. It's an inexpensive rifle and is a great platform as a start for customizing. It comes stock with a 26" barrel and in .308 you can shoot elk at 400, deer at 500+ and varmints/targets as far as you can see.