Which armor provides the most protection against regular damage?

Solution 1:

Against melee'ing aliens? Go Titan to take the hit or go Archangel to avoid it. Melee doesn't miss and hp is the only important stat.

Against plasma weapons? Ghost armor has 6 hp and 20% defense.

Most aliens have an aim stat lower than 100. If you can raise your defense to 100, those aliens cannot hit you with plasma weapons. Here are a few ways you can get unhittable against those aliens.

Anyone wearing Ghost armor can get unhittable by hunkering in full cover:

Ghost armor (20%) + Hunkering in Full Cover (80%)

Here is how each class plays the defense game to get close without hunkering:

Support (100%) : (Smoke Grenade can help other people).
  Ghost armor (20%) + Full Cover (40%) + Dense Smoke Grenade (40%)
Heavy (90%) : (Suppression can help other people).
  Ghost armor (20%) + Full Cover(40%) + Target Suppressed (30%)
Assault (80%) :
  Ghost armor (20%) + Full Cover (40%) + Tactical Sense with 4 enemies (20%)
Sniper (60%) : (need to add elevation to be competitive).
  Ghost armor (20%) + Partial Cover with Low Profile (40%)

Solution 2:

In this case, the 10 bonus defense is the same as the defense stat you start seeing on Mutons and their ilk.

10 Defense means 10% reduced shot to be hit, and has nothing to do with the potential damage spread. Unfortunately, this also means you're completely unable to notice when you're saved by Psi Armor's defense, compared to Titan, where you notice every time you're left with only one or two hp.

Skeleton suit also has 10 Defense, for what it's wroth.