I can't help you on what's the best offhand dagger because, truthfully, it will depend tremendously on the content that you are doing and your play style.
Here are some of the good options right there. Keep in mind the bonus that they have to offer.
---Oneiros Knife: This is the new X's knife as it increases critical dmg, has a very low delay (150) and 6 agi.
--Kila +2 (wind): This is your evasion tanking dagger. It has 9 agi plus a total of ~24/25 evasion when counting agi. Only negative I see is 190 delay.
--Kila +2 (fire): The attack dagger. It will boost your damage as well as it has modifiers for, in your case, your weapon skill. 9 str and 20 attack (not counting the att from str). 190 delay though
--Kila +2 (dex): The acc dagger. 9 dex and 15 acc (not counting from dex). 190 delay.
--Twilight Knife: the usefulness will depend on the type of drain you get. 190 delay. Quadruple attack 3% (kind of low but it's a boost)
--Triplus dagger: Triple attack 3% and increases triple attack dmg. Depending on your atmas this is really good. Has a nice 176 delay.
--Rapidus Sax: I found this dagger fun as it's easy to obtain and it replace my long lasting and loving sirroco kukri. It has 150 delay, 12 acc and 1% haste.
--Twashtar lv. 90: As a joke you can get. A joke because the dagger is amazing main hand. I think from what I remember it's just as good as Mandau is now at 90 after getting the boost to its ws.
The options are amazing and there are plenty unlikely in the old days where there were just a few.