I've said it before, I'll say it again: I didn't care for Dave Coulier as Venkman either, but in fairness it wasn't all his fault.
(*rant)
1. He was hired to Sound Like Bill Murray. That meant he couldn't impart as much of his natural personality into the part the way Murray, Music, or the voice actors of the other characters could. The fact that he refused to be a Bill Murray impersonator, and did it his own way, is a big part of why Music was so successful in getting out from under Murray's shadow and creating one of his own.
2. J. Micheal Straczynski, one of the most gifted writers in all of media (creator of Babylon 5 and current writer of Amazing Spider-Man), quit the show because the producers wanted to lobotomize Janine and make the show generally more “kid friendly” (ie make Slimer more important, put in the assinine Junior Ghostbusters). The writing quality went way down, but notice that in JMS written Coulier episodes such as “The Grundle” or “Janine You've Changed” Coulier comes closer to succeeding in the part.