Back when Blau first cam eout I was extreemly misinformed on it's stats. That being said, what you are about to read goes pretty contrairy to what I normally preech. Which in and of itself should be a lesson to all those who are taking the time to read this:
It's okay to be wrong, It's okay to change your opinion, and it's okay to have differing opinions.
As a thief, you end up in a couple situations, each with thier inherant needs.
If you are using one dagger, I.E. you are subbing War, Sam, Drk, Drg, Rng, you are likely doing the following:
War: Meriting, or Killing an HNM.
Sam: Killing a pop NM or HNM, or headed to an NM or HNM your LS already has claim on.
Drk: Killing an HNM that requires a stun once and a while(subbed does not get resisted) Or an HNM that has easily predictable/controlable AoE Weaponskills.
Drg: There are some minor merits to this Combo, but mainly for screwing around.
Rng: As an Akward, yet sometimes effective debuffer; particularly good for farming events.
So you got several subjob options, and several event options. Now you get to use your head. For Most things involving other people, you want to utilize that job trait called "Treasure Hunter" and enhance it. So most likely you should be using a Thief Knife. Even if it's only a 1% increase, it's still your responsability to make sure you are maximiszing your usefulness, for the group.
For all other times:
Mandau is the cream of the crop, and as a raw damage weapon outshines all others.
Barteradau is the Stage IV relic, it is a superb weapon, exceptionally high base damage, a decent delay, and no annoying Poison to complicate sleeping mobs when needed.
X's Knife adds a 25% bonus damage to all critical hits including Sneak & Trick attacks, unknown at this time if it still functions if offhanded, however it's got a pretty high base damage as is. When using any weapon skill, particularly Evisceration, it adds that bonus damage to the weaponskill when paired with SA or TA. Evisceration is a WS based on Critting and you can kinda Double dip on trailing hits.
Blau Dolch is probably the best Dagger of the choices for it's attainability. The Relics and the X's are nice, but good fucking luck getting em. Not impossible, but just fucking hard.
Stylet. Before the Thief Knife Fix(not a nerf, it was a fucking Fix) it was my weapon of choice. It's stats, when used on HNM, would in theory put it just a notch below Relic During SATA+WS, but would drop a little lower during TA+WS, or SA+WS.
For Thief/Ninja:
The best options would likely be:
1) Mandau/Ridill
2) Barteredau/Ridill
3) Mandau/Blau Dolch
4) Barteredau/Blau Dolch
5) X's Knife/Ridill
6) X's Knife/Blau Dolch
7) Blau Dolch/Ridill
8) Blau Dolch/Sirrocco Kukri
9) Blau Dolch/Stylet
Well that just about it in a nutshell.
Please note, if you're playing to not be a pest or a shitty thief, try and remember your role. You're not there as a DD at an HNM camp or Farming session. And you're not there as an MP sponge in a merit pt. Be responsible. If you're doing more damage as /war but sucking up 2x as much MP as you would subbing Ninja, you would be helping your party subbing ninja. Don't be a tool...
All and all, all these things do in fact maximize yoru character, but all the gains are minimal unless stacked up. I.E. you really think you'll do good damage with only a relic weapon? Or you think that Relic weapon will do significantly more damage when you have the gear to support it.
And for fuck's sake stop wearing those stupid fucking Purple JSE-50 Harnesses... They're not that great...