In The First Wave, the story of Steven and audio-only companion Oliver, and their travels with the First Doctor, comes to an end. It's a story which features the Vardans, which is a stroke of genius - they really belong much more to Season 3 than to the colour era, and somehow the sense of the story fits rather well to the era in which it was set, with Peter Purves and Tom Allen bleakly sparking off each other as death and destruction rage around them. I felt though looking back on it that there wasn't quite enough plot to sustain a full hour of audio.
The Five Companions is consciously a Christmas romp for us diehard fans, with Peter Purves again opening the story being chased by a Dalek. He finds other people here as well - his old acquaintance Ian Chesterton, Sara Kingdom back from the dead, a woman called Polly, another called Nyssa, and a young fair-haired man with a pleasant open face who claims to be the Doctor... also Sontarans, Daleks, and other nasties. It didn't make a lot of sense to be honest but I loved it anyway. I particularly noted the audible chemistry between William Russell and Sarah Sutton, who had played against each other previously in an earlier Big Finish story in the main sequence.
Finally, Army of Death brings the Eighth Doctor and Mary Shelley to a future planet equipped with complex politics, scientists playing with artificial life, and (until recently) two major cities. Mary Shelley is now far from her own background but comes over more as Leela than Victoria, with of course the obligatory subplot of her falling in love with the Doctor. It's well enough done, I felt stronger than the previous run in this series, and I was glad that the ending seemed to leave the path clear for more Eight/Shelley adventures.
In summary, I think all of these would require some knowledge of their particular branches of Who mythology to appreciate properly, with possibly Army of Death the least impenetrable to non-fans, and The Five Companions likely to end up as the most memorable.