I just ran across this challenge, but thought I'd enter one I did as a challenge to myself which I started in v .18 and finally accomplished after many iterations in v.90. My challenge was take an orange tank into orbit w/o using any of its fuel and return to the ksc landing strip, stock only, no refueling, ssto. The newly named Constellation, Jumbo SSTO 2 is a spaceplane capable of delivering a 36t untapped orange tank to LKO and returning to the KSC landing strip. http://kerbalx.com/sumguy/Jumbo%20SSTO%202 Here's my challenge proof video
Looks like it's up for Advanced Pilot Precision, Utilitarial Commendation, maybe largest ( 28.04 x 7.63 x 26.13, 250 parts ). Anyway it was a fun challenge, and I'm happy to get to share