DT's music is varied, and so is the musical background of new fans, so it's hard to say a song is a bad song to introduce someone to the band.
Some songs may seem too long and complex to start with, but I never had a problem with the longer songs from the start, in fact I thought it was cool at the time that these songs were longer and more difficult than anything else I'd heard. A lot of other prog/metal/rock bands write songs that are longer than average, even if still nowhere near DT's average. Just because people aren't used to DT's song length, doesn't mean they're not ready for it!
Some songs may seem too short and not representative, but this works for a lot of people too. DT have written a lot of amazing shorter songs, even if they don't showcase all elements of the band to people. Ween them onto DT with something catchy and easily digestible, then work your way up to the longer songs. Start with just the tip so they don't cry when you go all in.
And then other songs seem like a bad choice simply because they're not good songs to us, but that comes down to opinion. I've heard of people getting into the band from all kinds of different songs, even ones that people would suggest as joke answers.
So to answer the question, I don't know.