I'd say any ballad (Wait for Sleep, Space-Dye Vest, Hollow Years, The Spirit Carries On, Beneath the Surface etc.) would be a bad introduction. People who expect technical proggy stuff would be turned off, as well as people who want some heavy riffing and shredding. Many people who enjoy those ballads (and that type of music) right off the bat probably wouldn't appreciate DT's technical & heavy side that much. So it's a lose-lose situation.
I think the vast majority of DT fans were drawn in by either the prog or the metal side of DT. The ballads are a good added bonus and can be a nice change of pace if you enjoy them, but I usually skip them when I listen to DT (except for TSCO and Budokan Hollow Years, which are saved by the guitar solos, but both songs took a lot of time to grow on me otherwise).
I always thought that many DT ballads don't really fit in their catalogue seamlessly, oftentimes they stand out like a sore thumb amidst all their intricate, technical and complex instrumental passages and heavy riffing/shredding. This aspect reminds me of ELP, where Keith brought his modern keyboard arsenal and wrote 20 minute heavy and virtuoso prog epics, while all Greg Lake wanted to do was to sing 3 minute long acoustic ballads. Two completely different worlds, different sides to the same band, which didn't really match or create a cohesive whole. Radio listeners who heard Lucky Man might have thought that ELP was this really soft, delicate band (except for the ending, which is a great musical joke and it's an appropriate metaphor to show off the clashing, dissonant nature of the band), while prog fans could end up dismissing and ignoring all the Lake-stuff.