Depending on what day you ask me, I will no joke say Ghostbusters 2 is the superior film.
The biggest knock I've seen against it is that it repeats a lot of the same beats as the first film, which okay it does, but it does them better.
1. Vigo is a bigger presence in the movie - Gozer was more of a force of nature that the Busters have a single conversation with, Vigo is constantly working through his minion Janosz.
2. They bust more ghosts - In GB1, we only ever see them bust one ghost: Slimer, while in GB2 they bust a few, mostly during the montage.
3. The scares are better (the subway scene haunted poor Scottsman when he was little)
4. Venkman is more of a lovable asshole than a sleazebag.
5. And they got rid of the smoking.
6. Most importantly, it's all original. New plot, new music, new jokes. None of it is recycled, and it's ALL good. The actors and scriptwriters haven't lost a step!
Truthfully, I tend to view GB1&2 as basically two halves of a whole. GB1 has the benefit of being the first, and nothing Ghostbusters related will have an impact like that again, but GB2 takes what GB1 did and does an extremely good job with it. It's shocking to me that the movie was recieved poorly enough that it turned Bill Murray off from doing sequels (until Garfield 2) and basically killed any chance of a true Ghostbusters 3. The videogame is the closest we'll ever get, unless they want to do an animated flick and substitute Maurice LaMarche in for Harold Ramis. (Probably the only person on earth I'd be okay with them doing that with)