Oh it would probably be a matter of preference, but for me the best Fan Film is The Return of the Ghostbusters. I believe The Denver Ghostbusters kicked the fan cinematic universe off with Freddy Vs Ghostbusters and I feel their sequel Return of the Ghostbusters told an awesome story that really felt like it belonged in the main Canon. It was a very original story and it really showed just how far a Fan film can go. In my opinion it Both looked way better and was way funnier than the film Sony actually put out in 2016. I can't believe that a fan project could actually put a Big studio to shame. To top it off The Return of the Ghostbusters came out in 2007 and still looked better than ATC.
I have enjoyed all of the Fan Films that Followed, but some of my Favorites are.
Ghostbusters SLC
Ghostbusters 911
Ghostbusters Spilled Milk
The Real Ghostbusters ( This one was pretty neat. They took the Boogieman episode of the animated series and built on it. They also play characters that have identical characters to the original Ghostbusters, but are playing completely different characters.)
Ghostbusters UK
Ghostbusters Five Nights at Freddy's
I really wish I could have seen Ghostbusters SLC Chronicles or Eddie Lebron's Ghostbusters Generations. Unfortunately they were never finished. Those both looked great. I look forward to seeing more there are so many talented people that have put these out and shared them with the community. You can really see the love and effort behind them.