It's hard to go wrong at 55gr .223. That's where I started. However, ...
Case prep can be a real bear. I load 5.56, .308, .40, and 9mm, and IMO 5.56 (.223) is the worst.
.223/5.56 cases have one of the highest "stick-shun" coefficients of any bottle nose case. Surface area divided by case volume. I had lots of stuck cases in the resizer. Punches, hammers, and Navy Language. Finally found a case lube that worked - a 1:10 mix of 80W gear oil to isopropanol (a.k.a. iso-Heet) - both found at the Local Auto Store. Cheap. Spray it onto the cases in a 1-gallon Zip-Loc. Let it dry (in a well-ventilated place) or you'll get hydraulic lock. Search THR, it's somebody else's idea, and it works. Good luck to you.