Which nebulite should I mount on my weapon?

Well, for any monster with 20% or more PDR, the Ignore 18% Defence is better than the 6% Crit, and the PDR Ignore is better than 4% Total Damage on any monster with at least 25% PDR. Considering you're well into the fourth job range, almost every monster you'll be fighting (that you aren't killing in 1-2 hits) has at least that much.

Too bad the +6% Crit Nebulite isn't as good as the others. Honestly, I wanted to get the +6% Crit one just so my Wild Hunter would have over 90% Crit Rate with the Phantom's link skill.