By manufacturer: Most of the options you'll find are great. I usually order the 50rd LE boxes because you can get them online for about the same price as the 20-25rd defense boxes you'll find at gun stores...so that brings my cost down to 50-60 cents/rd. I go with Winchester Ranger, but there are a dozen other quality options. Speer Gold Dot, Federal Hydrashok, and Remmington Golden Sabre are way up there, as well as almost anything from CorBon.
By weight: There are basically three weights to look at for 40sw. Full weight (180gr), medium (155-165gr), and lighter/faster (135-140gr). There's merit to going with each of them, and the medium weight seems to be the most popular defense option.
A G23 should cycle any of them fine.