For The Shinies! A Guide For Thief

eminent dagger (7000 Sparks from RoE NPCs): Can't go wrong with this.

homestead dagger (30000 Bayld from Craggy Bluff in East Adoulin): A decent counterpart to Eminent Dagger, and will make Reives (should you do them) much faster. I still carry one in case I want to do Wildskeeper Reives. If you're short on bayld, spamming low level skirmishes can be effective, or trade in unwanted WKR/RoE gear to Runje Desaali in East Adoulin.

atoyac (Tchakka or Mog Kupon AW-WK): As it can be augmented with Prah Janimhar in West Adoulin to have Occasionally Attacks Twice, this dagger will be extremely useful for offhand on anything where accuracy isn't an issue. Other recommended augments may vary based on how you gear and play, but you can't really go wrong with STP, WSD, Crit Rate, or Crit Damage.

leisilonu +2 (Old Skirmish or Mog Kupon W-SCC): I would only suggest this because the Kupon is pretty easy to get. You'd likely retire it very quickly, even if you tried augmenting it (augmenting this sucks).

izhiikoh (Dakuwaqa, Foret de Hennetiel Delve superboss): A very well-rounded dagger, only surpassed these days because newer daggers do what it can and more. Delve requires 3 people for entry though, and Dakuwaqa isn't easy for beginners, so you'll likely need support from either experienced friends, a LS, or a capable pick-up group. Can also be bought from Forri-Porri for 200k plasm, but that's a steep price for a replaceable dagger.

Ipetam (Alluvion Skirmish Rala, or Mog Kupon W-ASRW): With the RoE Intermediate, you could get one of these solo relatively easily. Requires good augments to beat Izhiikoh, so you might want to just use it in the short term and throw some cheap stones until you replace it.

jugo kukri +1 (Vedrfolnir, Cape Teriggan Unity NM): Amazing offhand dagger, though you'll probably replace it with Taming Sari or Shijo. Massive jump in difference between this and the NQ; don't settle for the NQ, though dual wielding them is actually pretty decent. Could probably get this with shouting for help, as the NM is popular and not terribly difficult. Unity Event going on this month as well, so you've got a greater chance for getting this.

Sandung (Oboro JSE Weapon): This dagger is pretty expensive these days, and isn't particularly useful unless you can fully augment it (requires beating Rhapsodies of Vana'diel), further increasing the cost. No real long-term use either, as the accuracy augment doesn't compare to it's low benefits. Pre-augments, it's pretty awful. With augments, it's fine, but it'll still lose to a Taming Sari. I would highly recommend against making one since your gil can be better spent.

ternion dagger +1 (Mephitas, Garlaige Citadel Unity NM): Not that great, as it lacks DEX and has lower skill than the rest. Probably the weakest of these daggers, but it's still better than the starting daggers.

Odium (Plouton, Vagary superboss): A good mainhand dagger, and the added jobs are a nice bonus. Roughly par with Izhiikoh. Added death is pretty rare.

shijo (Amymone, Escha - Ru'Aun 125 NM): Path D is ideal, though bear in mind that the extra 5 DW may hurt you if you're over the amount you need. Great to offhand with Taming Sari main.

taming sari (Sinister Reign; requires beating Adoulin Missions): Best non-RMEA dagger. Only reason not to use this would be for utility purposes. Requires a bit of luck to get, though, and more so to get the max augments. With crap augments, it's still better than most other daggers. This should be your eventual target if you don't want to make a RMEA. Using two is an option and a very strong combo, especially for the accuracy.

Skinflayer (Strophadia, Reisenjima 135 NM): With good augments, it can potentially rival Taming Sari, but more often than not it falls short. Probably easier to get though.

malevolence (Sinister Reign; requires beating Adoulin Missions): Best Aeolian/magic damage dagger, not impressive melee damage though. Even with crap augments it's pretty great.

nanti knife (AH or crafted): Doesn't really provide much, especially for the cost. Save your gil for better investments.

blurred knife/+1 (AH or crafted): There are some people who might suggest this, but for the most part it's not really useful for THF (only provides Haste and some acc). In most situations, you'll be getting Haste anyway, either from players or support trusts. The only places you might not would be something like Salvage (assuming you're not /RDM).

Everything Else: The rest either come from harder content than better daggers, or are otherwise not worth the investment. If you do happen upon an improvement by chance (like from the daily Goblin Box), then by all means use it until you can replace it.