There were several contenders for the spot. A close second was broken compass by two little mice which is a fantastic game and is easy to understand and has plenty of resources. it really was kind of a coin flip.
However I’m gonna have to say Ghostbusters. Keep in mind I haven’t actually played the game, I’ve only read it, dissected it, and discussed it with other people. So why would I recommend that as an introductory game?
The answer could be that it’s easy, simple, and enough people have seen the movies that the genre is evident. However that’s only part of the equation.
The real thing that makes it such a great introductory RPG is that it contains so many elements that you will find in other RPGs, and in several cases it was the first one to incorporate them. Dice pools, skills, a meta-currency (which doubles as your health no less!), a chance of a result orthogonal to skill roll, and some other gems can be found within.
It can be a fun and light hearted introduction to lots of concepts that you will see in other RPGs down the line.
Who you gonna call?