.222 Rem.
Although not as common as it used to be, it can't be beat. Less powerful-and usually more accurate than the .223, it is a dream to reload. Case life is great, and if you're loading anyway, you can load it down to fairly quiet plinker levels.
Not just sub MOA groups, but sub 1/2" groups are fairly common with this little sweetheart. I dare anyone who owns one to downtalk it; cmon, I dare ya.
jd