Can only help with lower ones! That's all I've played as Orc fighter or other melee..........
NG--doesn't matter.
D grade--brig if you can afford multi-million dollar helm otherwise go mithril heavy.
C-grade--still not that important though you can get Full Plate reasonably cheap.
B-grade you'll have to ask someone else.
Generally an orc fighter is a 'pure melee' imo. That means he/she stands close to mobs--deals damage--and gets hit in return.
That's a good place for heavy armor. At least through C-grade I have yet to encounter normal pve play while leveling where the armor matters much. Most are 'competent' enough and i think it's better to spend your adena on a better weapon than a 5 million adena d-grade helmet.
my 2 cents.
If you can't afford brig helm, it's not a big deal. You won't be in D grade for all that long.
For C grade full plate is probably the best, but m.def bonus on composite with shield is also quite nice. (EDIT: should you go the one-hander + shield route)
B grade, blue wolf heavy is the DD set.
A grade, I've seen a lot of destroyers in majestic, and it has a nice bonus too.
S grade, there is only one choice; Imperial Crusader.