Re: ammo for a 1 in 7 twist 223 ar15
The best is what you load.
The best isn't necessarily heavy.
Twist + chamber is more important than just twist.
"Cheap" ammo often means inconsistent powder charges, cases with wider internal capacity variances, FH variances, neck tension variances, excessive runout. In short, you could slap a 69 SMK (generally a great bullet in most any barrel) on top of a case with all of these other inconsistencies and still have a crap load.
Summation: if you want it to shoot really well - you need to load it.
Otherwise buy some Black Hills Ammunition 'Blue Box' in 69 or 77 grain.
Good luck