The Axis of Insanity: I felt this one was a bit average, really. Peter Davison gets some very good moments, both as the baddy pretending to be the Doctor and as the Doctor reflecting on what it's like to be a Time Lord, but apart from that it didn't do a lot for me.
Arrangements for War was much more engaging, a story of the Doctor trying to get Evelyn's mind off recent tragic events and ending up getting over-involved in a local political situation. Indeed, really rather too involved for my reading of how both the Doctor and Evelyn normally operate. But the whole thing hung together well, with a grim and tragic story which yet found some redemption at the end.
I actually rather liked The Roof of the World; even though the good bits are all stolen from other Doctor Who stories (apart from the nods to Lovecraft) I liked the development of Erimem's character, and Davison and Bryant got good chances to do their stuff; plus the setting was on the whole very well depicted given the audio environment. I was rather astonished to find out the back-story of the author after I did a little googling...
There was quite a lot to like about Medicinal Purposes - Baker and Stables on top form, joined by David Tennant as "Daft Jamie" and some sympathetic and attractive minor characters - but in the end I felt there were two big flaws with it. One was the means and motivation of the villainous Dr Knox - there seemed to be at least three explanations for what he was really up to, none of which fully made sense, and the story lacked resolution. The other was the Doctor's undisguised admiration for Burke and Hare themselves; again, I felt this totally out of character even for the Sixth Doctor.
I'm coming to realise that the Big Finish audios are a bit hit and miss; but then, so was the classic series, and so is the new version too. Sturgeon's Law, I suppose.