One of my favorite DW audios is Big Finish's Doctor Who Unbound: Masters of War
The story takes place in an alternate universe. The doctor is played by the amazing David Warner, and his companion is Alister Lethbridge-Stewart!
I love that the tardis is occupied by these two (occasionally grumpy) old men - engaging in adventures better suited for youngins like 11 and Pond.
It is a great dalek-centric story, with daleks that are actually intimidating for a change. There are some surprising twists and turns. I found myself attached to the supporting characters, and the ending is quite satisfying.
If you're looking for a one-off, self-contained DW audio adventure, this is easily one of the best.