With my club, it's always been top 5 hitters/money made. I have almost always gotten a drop if I'm actively hitting the party and not unloading.
Being top plunder doesn't give you a better chance. Having the most or more actions doesn't give you a better chance.
Something does give a better chance in my mind. Same player in my club has gotten four pugs out of five parties. You can check the history, but the breakdown is: Hits-32-77 Plunder under 50m and never top plunder Never the last hitter Did at least 1 of each action the the party I feel like there has to be something, but my club is a small sample to get a read from.