I didn't respond to this already? Shame on me.
In no particular order...
The Unseen The first one I ever saw, which always helps with a "sentimental favorite" tag, but also it's a good one, with some great gags and development with Eduardo and Kylie (the #2 Ghostbuster couple to me, since Venkman and Dana were a movie-only thing. You all know who I have at #1)
The Crawler Speaking of which...while I was a tad disappointed to see that Egon and Janine were "reset" like "Janine You've Changed" never happened, and this episode was like a not-quite-as-good-but-still-good remake of it, but Egon saying "Give me some sugar, Baby!" and Bug Janine's surprise betrayal of Cojilla were classic in their own right.
Deadliners Gets here for being probably my favorite villains of the series--the Vathek were creepy and conceptually neat, and so over-the-top that it made them circle around to being kind of goofy. Plus another of the funniest bits of banter in the series, after Eduardo recognizes Crainiac:
You...read JN Kline?
You read?
Darkness At Noon Strong opening story, good moments for everybody. "Why? She's legal!" The fact that it appears that Egon and Roland whipped up four all-new, never tested proton packs, a new ghost trap, AND remodeled the ECTO-1 in like three hours was the only real eye-rolling moment to me.
Back In the Saddle I thought there was just a couple of miscues in this story--I thought Ray was uncharacteristically rude to the new kids at first, not to mention why the sprock was he wearing one of Egon's flight suits--but 99% awesome is still awesome.
Sphinx Well, everyone knows Egon's one of my favorites, and this was a great spotlight episode on him.
There's so many great episodes in this series, but I gotta put those at the top.