City of Death for me. Rounding out the top 5 would be Talons Of Weng Chiang, Robots Of Death, Seeds Of Doom, Ark In Space.
Wolfpanther
The Bridge by Enza Gandolfo
Tales from the Security Kitchen - who doesn’t want to learn more about the Security Kitchen? For example, why is there a security kitchen?
No Complications - explores the back story of that one Ogron who speaks English much better than the others.
The Adventures of Belal - The doctor’s little sidekick from Death to the Daleks in a series of adventures involving various menacingly tiled floors.
I'm a bit of a late comer to Big Finish and make no attempt at keeping up with everything but I pick up bits and pieces, often when on special. On of my favourites recently has been The War Master series, I've found that to be consistently really good.
There was a recent Youtube video from Building Beautifully that addressed this topic https://youtu.be/QATMxSQWyZ0 that's worth checking out. Also plenty of discussion in the comments there.
I recommend the episodes of the Behind The Bastards podcast where they read through Ben Shapiro's novel, they are hilarious.