Benchmark and Varget are both very very good.
I second the Varget. Several websites (loaddata.com) have all the types of powder combinations for .223 you could potentially use.
Benchmark 55g and under
Varget 69g and over
Varget 69g and over
I used IMR 4064 and my favorite for both .223 and .308 is Varget. Varget is a great powder and versatile. It kind of all depends on the bullet you select as well. Different bullets will perform slightly better or worse with different powders. You really can't go wrong with the two I mentioned though. The have both performed very well with several different bullet designs and weights. Just an FYI H&H had 40 lbs of Varget on Thursday afternoon when I was there. Hard to find, so you have to jump on it when you get the chance. I can sometimes find it at the gun shows as well.
