This isn't to say the other releases haven't been good, the worst I can honestly say is that I thought The Highest Science wasn't as funny as the book but it was still entertaining. Rather, its just that all the ingredients were present in this one in just the right ratios and just the right places.
You've got the in medias res beginning as we join Benny and the Doctor at the end of a mainly unseen adventure. You've got Benny, Chris and Roz together, which was the definitive TARDIS team of The New Adventures as far as I'm concerned (I never got on with the guntoting space marine version of Ace). You have a discussion of the Doctor's morality, albeit not a terribly compelling one, with the serial killer Zebulon Pryce. Most of all, though, there's the scenes in the Overcities of 30th century Earth teeming with aliens and sly little reference like the Birostrop character.
One of the big selling points of the Virgin era was that the books had a pretty solid sense of what the future looked like in the Doctor Who universe but, sadly, not many of the books Big Finish have been able to adapt have been ones using that solid future continuity. What little we've seen of that future in the adaptations have been isolated outposts of the Earth Empire (Love And War, The Romance Of Crime) or random archaeological expeditions to the edge of space in that era (Theatre Of War, The Highest Science) with this being the first time the series has done the sweeping space opera I remember from that era.
Given that the adaptations seem to be ending with Cold Fusion (which I am so very much looking forward to) its nice they got in one like this before the end, a real nostalgia fest for the majestic height of the novel line.