I would say there are two practical choices.
223- cheap to free brass, cheapest projectiles going, light on powder use and long barrel life, all the round you need out to 300yds
6BR- great brass selection although pricier than 223, sips powder, excellent bullet choices, superior accuracy to almost everything out there, great barrel life, very easy to load, and great choice of barrel twist. I personally wouldn't go with the fast twist to shoot at 300 or closer, but I will probably be the only one to tell you that. Fast barrel twist is hard on bullets and they wear faster and takes speed away from bullets that don't need the fast twist. If you were going to shoot 600+, then a fast twist would make a lot of sense. I have barrels for both, and strongly prefer the 1-12 for 75-87gr bullets.
Otherwise there are lots of good choices, these two just make the most sense to me.