Ultimately it looks like combat is simply too deadly in optimized d&d to rely on defense at all, ever. Winning is about investing in going first and still hitting hard enough to end combat before the other guy manages to. Short of some spellcaster's tricks where lower level spells can be used to increase defense without trading away wealth or offense, true protection is unattainable on a practical scale without sacrificing all chance of winning.

This does not mean that all hope on defense should be abandoned, but rather that things with minimal effects that always work, for minimal costs, will be the best defensive items in case combat makes it to the second round.

As attack bonuses are widely variable, AC bonuses are not a reliable form of defense for a low investment.

Low cost protective items also tend to provide bonuses other than AC bonuses, because WotC had a way of "balancing" prices for AC boosting items, but not for other protective items.

The short version is improved initiative is better than armor class, and the gap only widens as levels increase. Things that provide flat % bonuses are next best because they will almost always have the same reliable (if minor) effects. Armor class boosts are simply too high an opportunity cost for characters who must divide their wealth between offense, defense, and initiative/extra actions.