Who would you consider the "leader" of the Ghostbusters?
My natural first instinct would be to say Peter since Bill Murray is first billed, Peter was the one who suggest to Ray that they (after them and Egon got kicked out of Columbia University) should go into the business for themselves, and is sort of the main and most natural "salesman" of the Ghostbusters. Also, at the end when Louis Tully has part of the terror dog statue covering his head, Peter directs them to "go check on that little guy".
On the other hand, I you can also argue that Egon is more of the "leader" since he's obviously the most mature, cautious, and serious minded of the team. He also gave crucial information like the origins of Dana Barrett's apartment and its connection with Gozer, as well as warning Peter and Ray the dangers of "crossing the streams".