Dark Knight: Guide by Raenryong/Weapons/Great Sword

Level 1: Rusty Greatsword - Well, it's better than the Chaosbringer at least...
Level 5: Deathbringer - Get this; do not bother questing it however, it's really cheap on the AH.
Level 10: Claymore/+1 - Does the extra DMG outweigh the attack on the Deathbringer? The +1 may do, but I would say the NQ probably doesn't. No harm in sticking to Deathbringer here.
Level 13: Vulcan Claymore/Flame Claymore/Burning Claymore - The dmg gain here is very significant as compared to the Deathbringer, so make sure you upgrade here. Note you can't sell the Vulcan Claymore back if you use its enchantment.
Level 18: Braveheart - NM drop; adds DEX and +5acc as well as sporting a high DMG. There is no reason not to get this weapon.
Level 20: Mercenary's Greatsword - Higher DMG and +atk than Braveheart, but no accuracy. Your choice; obviously better if you can land most of your hits.
Level 20: Two-Handed Sword/+1 - Very high DMG but little else. Braveheart, Mercenary's Greatsword or one of these are all solid choices for this level.
Level 24: Barbarian's Sword - High DMG and a very nice +attack... however, the -5acc at this level is killer. Maybe worth using if you're using sushi and have a bard, but otherwise, don't bother.
Level 25: Windurstian Sword/Federation Sword - Really nice DMG, +attack, and the +MP is nice too. Worth using.
Level 27: Prominence Sword/Inferno Sword/Hellfire Sword - High DMG and additional fire damage... very nice weapons.
Level 31: Demonic Sword - High DMG, additional darkness damage. It's nice.
Level 36: Mythril Claymore/Fine Claymore - Powerful greatswords if you can catch them on the AH; they're not made very often and have challengingish crafting requirements.
Level 39: Ice Brand - So many greatswords in this level range have high damage and then a nice additional effect. Hmm...
Level 41: Greatsword/+1/Gust Sword/+1 - Hard to find on the AH, but the best at this level. The Gust Sword is just a Greatsword (original name) with an additional effect you're unlikely to use, get whichever is cheapest/available if you can!
Level 43: Royal Swordsman's Blade +1/Royal Swordsman's Blade+2 - Nothing revolutionary, and again very hard to see available. Greatswords as a whole are scarce throughout these levels.
Level 47: Faussar/+1 - DMG upgrade, hard to get ahold of. Same conditions you've been facing the last few weapons.
Level 50: Cobra Claymore - Beastly greatsword for its level and easy to get!
Level 53: Glorious Sword - Hard to get since it drops from a very tough NM, although it is an incredibly good greatsword.
Level 54: Darksteel Claymore/+1 - Again, very elusive. Greatsword users have such a hard time in this level range.
Level 57: Darksteel Falx/+1 - Relatively small upgrade from the darksteel claymore, and again incredibly hard to find on the AH.
Level 60: Zweihander/+1 - Not too rare on the AH, surprisingly -- also very good.
Level 62: Arondight - Drops from Cancer, a powerful greatsword for its level and very aesthetically pleasing as well. Whether it's worth all of the effort is up to you.
Level 66: Mythril Heart/+1 - Additional effect of Dispel and powerful to boot. A very nice greatsword; not TOO hard to find on the AH either.
Level 70: Balmung - Drop from Fafnir, but luckily not rare/ex! An obscenely good great sword, its only flaw being that it reduces the effectiveness of Souleater substantially. Use this and love it; it also looks awesome!
Level 70: Crude Sword - This should definitely not be used in exp over Balmung, but it is quite handy for a certain floor of Limbus because it deals BLUNT damage with its melee hits (although the weaponskills are still slashing).
Level 72: Martial Sword - Do not use this.
Level 72: Subduer - Depending on your luck, this can really suck to get. It drops randomly from some KSNMs, and if you're unlikely could take many tries to get (since it's rare/ex). That said, it is easily the third best greatsword in the game when unlocked.
Level 73: Perdu Sword - Obtained by paying 48,000 IS with a Second Lieutenant or higher rank from a Sanction guard. Has a powerful hidden effect; this will easily beat Balin's Sword but will probably lose to Subduer and almost definitely to the Algol and Naglering. Still worth it if you have IS to burn or bad luck getting the other three.
Level 73: Balin's Sword - This will produce higher weaponskill damages than the Balmung, but will inflict less DoT in likelihood; however, it also does not hamper Souleater damage. Use this in endgame battles but the Balmung is probably better for "softer" opponents than gods etc. Subduer of course beats both options (for "softer" things, Balin's Sword will do marginally more weaponskill damage) and is only bested by Algol and Naglering.
Level 73: Gold Algol/+1 - Do not use these either; you do not need those accuracy boosts so much at this level, and it lacks compared to the stronger greatswords.
Level 74: Naglering - The new king of greatswords, tied with Algol. Huge attack bonus, accuracy bonus, and high delay which is actually a boost since it allows you to make a low "x hit" build. Hard to get though, from high-tier ZNMs. Please see notes at the bottom.
Level 75: Algol - The king of greatswords, tied with Naglering. High base damage, +Attack, an additional fire attack proc chance and... a chance of Triple Attack (3%). This can skyrocket either your TP or your weaponskill damages depending on where it procs. The catch is it drops exclusively from Cerberus, so unless you're in an HNMLS there is very little chance of ever seeing this beauty. Do not worry, however; Naglering is accessible by just about anyone and is debatably even better!

Notes on Naglering
In order to take advantage of the opportunities the higher delay gives on /sam, you have to either;

• Eat Carbonara, TP and WS normally (assuming you have a Rajas Ring and a Brutal Earring)
• TP and WS in +8 Store TP (+2 extra than Rajas Ring and Brutal Earring) -- this can be achieved with items such as Attila's Earring, Chivalrous Chain and Ecphoria Ring. Remember you must also WS in +8 Store TP from gear.
• TP in an Askar Korazin and WS normally -- a fairly large accuracy loss from not using Haubergeon so bear this in mind.
• Rose Strap is wonderful for this kind of idea. Yes, it is entirely worth giving up Pole Grip for and is probably your best option to create this build.