David,
The vast majority of guys that wear level IV armor on a regular basis are going to wear whatever is issued with the plates. I've never met a person that has bought and tried multiple types of carriers and worn them extensively in an effort to determine which brand is best.
If I were having to purchase one I'd pick any decent brand that has the capabilities you're looking for. I prefer next to nothing on my armor. Two M4 mags and a sharpie is about my limit. I also have a flapped pouch for throwing schit in. Namely my cell phone. MOLLE would be essential of course.
Outside of that I wouldn't want anything. I also prefer a totally separate armor to belt system. Not an LBV type that is assembled as one.
Garand Thumb has some good reviews if your'e so inclined.
