Bad Taste is his first movie, and in my view perhaps a little too much of a rough and ready beginner's effort. Braindead, his second, is a comedy schlock horror masterpiece right up there with the best of the genre such as Evil Dead 2 (still, I believe, the record holder for the most fake blood used in a film).
After that, there's the Muppets-baiting Meet The Feebles, which is another comedy schlock horror. I cannot help but feel this oversteps and misses the mark somewhat by being too revolting and juvenile. Nevertheless, it's certainly one to watch for fans of that sort of thing. I think I read that Jackson was very frustrated with trying to find backing to make his movies, hence firstly the plot, and secondly that maybe too much of that frustration seeped in.