It shouldn't necessarily be a 1st level module. You want the experience to portray what D&D is like most, so picking a level players will spend a lot of time at is better. I would recommend level 7 or 8 for this. The subject matter, or plot, of the module should be quintessential if possible. You need to ask yourself what is your personal favorite, though, and show this to the players. Don't pick a module you aren't familiar with, because that will affect their experience too.