There is no pat answer on what is best, you'll need to test what shoots best in your gun. I've found the 50 gr Hodgdon v-max to work well in varous .223's and a .221, but some guns shoot it more accurately than others. You might find your 1-7 twist AR prefers a 75 gr.

My 218 Mashburn bee contender really preferred the 45 gr Sierra and was ho-hum with most other bullets.