So here's the question - are you leaning more in the direction of the armor simply making you extremely durable and able to stay in the fight, or do you want it to serve as an extension to the theme and "flavor" of the character? Invulnerability is kind of the baseline, IMHO - a mixture of defense and resistance, with some self-buffs a la invincibility, and a self heal when the chips are down, (or used proactively to expand your HP pool). On the flip side, you have more "stylistic" choices like dark and elec that you mentioned. Finally, there's my current favorite - radiation armor, which while lacking defense, gives you absorption and a ton more endurance to work with, (so if you want to dip into various power pools to further supplement your repertoire, that extra end use is accounted for).