Dark Knight: Guide by Raenryong/Head

Level 15: Enyo's Mask - If you do Salvage, you might have this. Not a big loss if not.
Level 24: Empress Hairpin/Emperor Hairpin - If you can get these, you will be using them a long time.
Level 25: Phobos's Mask - If you do Salvage, you might have this. Not worth the effort over the hairpins though.
Level 30: Dandy Spectacles - If you can be bothered putting them on for the right time of the day, they are better than the hairpins. I wouldn't consider it worth the effort though.
Level 35: Deimos's Mask - If you do Salvage, you might have this if you are very lucky. Not worth the effort over the hairpins though.
Level 39: Sinister Mask - Decent for weaponskills, but obtained from a very elusive NM. Probably not worth the effort.
Level 41: Voyager Sallet - Can be awful to get but is your best head piece for many levels by a long shot. Worth the effort, but could be a very painful camp.
Level 43: Walkure Mask - Not better than Voyager Sallet for TP or WS, but a very solid head piece if you don't have one. Better than the Valkurm Emperor head items.
Level 43: Valkyrie's Mask - Marginally better than Walkure Mask but not worth the extra cost unless you have money spare. All other comments on the Walkure Mask apply.
Level 50: Storm Zucchetto - Very rare item and slightly better than Voyager Sallet for TP, worse for WS. Not worth the effort, but if you have one and no Voyager Sallet...
Level 59: Sipahi Turban - About on par with Walkure Mask/Valkyrie's Mask for TP, slightly better for WS. I wouldn't bother getting one if you have a Voyager Sallet or Storm Zucchetto, especially since at level 60 there is...
Level 60: Chaos Burgeonet - AF helmet. Very good for Dark Magic macro, WS, TP at this level and Souleater-related stuff. Top quality helmet, definitely do not go on without it. Voyager Sallet and Storm Zucchetto are better for TP, but you should not skip this item -- especially if you do not have these. Valuable AF+1 armour later.
Level 60: Patroclus's Helm - Drops from a very obnoxious NM, but can situationally be the best TP helmet until 69. The -enmity, although not amazing, is definitely quite helpful at this levels. Not worth the effort to get it, but can be very powerful.
Level 70: Optical Hat - Best TP head for this level by miles, and a very good head piece for Guillotine depending on the rest of your build. Can be very time-consuming, frustrating and difficult to get, but well worth the effort. Drops from Hakutaku, 100% drop.
Level 70: Maat's Cap - If you have this, it makes for a very nice weaponskill head used properly.
Level 71: Gnadbhod's Helm - Very nice weaponskill head and worth using almost forever if you don't have Ace's Helm or Hecatomb Cap amongst over pieces.
Level 73: Hecatomb Cap - Immense weaponskill piece and has a fair bit of accuracy. Ultimate weaponskill head for greatsword weaponskills, and can be very potent for Guillotine if it fits into your build. Very difficult to obtain however.
Level 74: Ace's Helm - Crazy piece. Ultimate TP piece by miles and very good for weaponskill. Insanely difficult to get though... good luck! Drops from King Vinegarroon.
Level 74: Chaos Burgeonet +1 - Dark Magic/WS/Souleater enhancement piece. Not good for TP at this stage in your DRK progression.
Level 75: Ares's Mask - Can be very nice for Guillotine situationally, also a decent TP head in certain situations where accuracy is low. Very difficult to obtain.
Level 75: Askar Zucchetto - Slightly better than Walahra Turban on DRK, but a lot of effort for this piece for such a minor upgrade which won't always even be an upgrade.
Level 75: Walahra Turban - TP piece for a VERY long time in likelihood, and also a very good headpiece for zerging. Nice and easy to acquire.