In my opinion, the best is a very subjective term. I think any of the big 30's to 338's work well on elk at those ranges. 300 RUM, 338 RUM or improved versions of it, 338 LM ect...The important thing here is accuracy, shot placment AND bullet proper bullet selection.
As far as scopes go......So many of them work so well. The lupy mark 4's to Nightforce are all top performers. Again it boils down to application. Make sure what ever you use has either adjustable turrets or hold over marks on the reticle. The leupold TMR works well for these calibers to 800-900 yards. The Night force NP-R1, NP-R2 and MLR also work very well. I prefer the MLR with the Mil radian turrets as I do all my adjustmenst and calculations in Mils.