What bullet (and load) shoots best out of your rifle? And, what are you going to be shooting? paper? varmints/light game? heavy game? all?
The 168 is a target bullet. The 150 is general use, 180 for max penetration, and 165 is a good compromise between the two.
Also, how good is good enough? A rifle that shoots 2 MOA is good enough for deer and other big game at any range you ought to be shooting game at, but won't win 600yd matches, most likely.
Most rifles will shoot different weight bullets to different places, sometimes several inches different at 100yds. Some rifles will put them fairly close together, others won't. You should try out all available weights in your rifle, THEN think about which one is best.
As to which one will be best for 300yd elk? The one you can make clean shots with. A 150gr hunting bullet will do the job, if you put in the right place. A 180 will carry through a bit better, but it still has to hit the right place. 3inches high at 100 will put you about 8-9 inches low at 300 (depending on the exact load). And then there's the wind to consider, which can put you off by quite a bit if you don't judge it right. Your skill compensating for wind, as well as drop can make the difference between a clean kill, a clean miss, a minor wound, or a gutshot elk.
Mostly I shoot 150s, and 165s, and sometimes 180s. If your rifle shoots them well, the 125-130s make dandy pest bullets (for larger pests
).
Try some of each, and see what you and your rifle like best.
There is no magic bullet, btw....