Anything premium/bonded/mono from 120-150 gr that your gun shoots well. Standard cup and core will obviously kill an elk as well, but by going to a more engineered bullet I can go down a bit in weight and gain some velocity. From a 24" barrel you ought to be able to get around 3100 fps out of the 120 gr Barnes TTSX which I know first hand will kill an elk just right now. By contrast a 150 gr bullet would be going around 2800 fps.
This pretty much sums it up.
