Hopefully I didn't miss it. Does the OP reload?
I'd have to say 264WM here since we are discussing this on a hunting forum. Like Grey, I'm also a certified 6.5 "nut". For the hunting fields, I love my 6.5WSM; for the range, my 6.5 Creed gets the nod (well, at least when I take a 6.5 with me
If you reload, i'm not sure if there is a wrong answer. Performance wise, the Grendel kicks is off in dead last. The 260, 6.5 Creed & 6.5x47 are nearly identical. Then comes the 6.5x 284 with another 200fps; the 264WM & 6.5WSM adding another 200-300fps over that & STW rounding it out with another 150-300fps on that.
You have a velocity window of 2600-3400+ depending on which cartridge you choose. Not a bad time to look into the 6.5's.