I’d probably say Inquisitor, not because I don’t like it or anything but because I feel it does the least to carry itself out of all the masteries. There’s only one “core” spam skill in the whole mastery (FoI). If you want to go Runes, you need some filler for sustain cause they won’t help you in that regard. If you want to go guns, Inquisitor doesn’t provide any attack to use with them. I guess it’s a similar case with Nightblade’s dual wield but at least nightblade has SS, ABB and PBlades as other (possibly spam) build options and does a lot to carry them on its own. Moreover, Inquisitor’s core defensive pillar (Seal) forces a stationary playstyle which clashes with the potential kiting playstyle Runes or guns might incentivise and is real iffy for engagement on melee builds and disengagement on ranged builds.
There’s some much needed RR in Death Sentence, the consistent DR on Horn is nice, if short-ranged, and the defensive benefits of WoR are not to be understated but overall I have very few Inquisitor builds where Inquisitor is the star of the show. Often the star is something else and Inquisitor just provides WoR and some RR to support it. The builds where Inquisitor is supposed to be the star of the show have generally not performed as consistently for me as I would have hoped (e.g. Pierce Rune Infiltrator), with the exception of FoI. Though, admittedly, I haven’t done some of the more vanilla stuff like DW ranged Purifier.