Personally, the only two armor pieces that make a big difference are the 25% melee damage shoulder pieces and the 25% storm speed leg pieces.
However, I don't run a lot and don't melee as a play style. So, they are of no worth to me.
If I'm an adept, engineer, vanguard or sentenal and feel I need the extra power boost then blood dragon armor is litter literally the best armor with it's 15% power and 10% shield bonuses.
However, you can do 13% power and 8% sheild bonuses with n7 armor pieces and have the option to customize the colors and actually see sheppard's face.
On that note, aesthetics become more important to me, as watching flashing lights while sheppard talks is boring.
So, I go with the visor and armor combination that gives me 15% sheilds and 15% health. I'm not too keen on the legs that give 10% health bonus, but all around, I think it's the best combination of looks and functionality. It's the Sentry interface, Asymmetric Defense Layer, Shield Harness, Heavy Damping Gauntlets, and Life Support Webbing. When I'm on a mission that requires a helmet the bonuses drop to 10% shields and 25% health, so that's nice too

However, as I've stated before, you could play with the default n7 armor and make it through insanity if you preferred looks over anything else.