If fugues in symphonies can be included, then it would be a tough choice between Shostakovich 4th or Finale of Walton 1st.
hmmm...having finally listened to it tonight, I would personally prefer the Shostakovich. HOWEVER, I am feeling generally rotten tonight, and am feeling a bit of canon-fatigue at the moment.
I have warmed to one or two of the Piazzolla-fugues. In particular to one called "Fugata", on a disk called "Soleded 1969,1974", from a 10-cd box-set of his stuff released recently. (I relistened to several tracks this week after finding I quite liked "Oda para un Hippie" when it came on in shuffle mode a while ago).