I think the way to start your evaluation of chest rigs is to ask yourself what do I want to have at hand quickly and what do I use often? Anything you don't need quickly or don't use frequently can go into a pack. From that point you can determine what size pouches that you want on your chest rig. The second thing to keep in mind are the straps of your pack. Some chest rigs like the U.S. Army TAPS have pouches that go back rather far on the side of the wearer and that can interpret the straps of the pack making the pack less comfortable to wear and the pouches that the straps are covering up harder to actually use. The third consideration is whether the majority of the time that you're wearing the chest rig whether you'll have on a pack and/or belt kit, that will determine the need for things like six magazines on the chest rig vs. three or four and again what general purpose pouches that you'll need on the chest rig.
I am not a fan of the MOLLE chest rigs, while customizable they are bulky, ultimately heavier, and far more costly once fully set-up than just buying a turnkey sewn down fixed pouched chest rig.
The Velocity Systems UW Chest Rig IV is the gold standard that most chest rigs are measured against and for good reason. The Velocity Systems 5.56 Hybrid is a close second in popularity, for those who want a bit more customization with what they put on with the side/flank general purpose pouches. One of the Chest Rigs I've been looking at lately that looks interesting, but I don't have any experience with yet is the Citadel Defense Spectre Chest Rig and I've been thinking about picking on up.