What's the best offhand for maximum Mordant Rime damage?
Are you asking what's the best dagger to offhand if you gonna spam Mordant Rime?
Or are you asking what's the best dagger to OH if you want to ignore the overall damage and just check WS dmg?
Leaving aside Air Knife (which is BiS) the two main options are Ternion Dagger +1 (R15) and Perfect Taming Sari.
The former is the better option in general, but if you want to consider ONLY Mordant Rime damage, then Perf Sari is slightly ahead.
This difference is like 2 digits with AM3 down, and a bit larger with AM3 up.
I am of course assuming R15 Carnwenhan equipped MH.
Edit:
Surprisingly enough R30 Gleti is behind. Gleti might have an edge in uncapped situations though, truth be said.
I can see Gleti being a better option for people who want to MH Tauret and spam Evisceration, I suppose. Haven't checked how it performs against Twashtar OH, but I wouldn't be surprised to be see Gleti pulling ahead as OH for Tauret's Evis spam.