Hard to say. They pump out music like mad, and every album can be an entirely different genre.

My personal favourite is Polygondwanaland, though.

Depends on your style. Most albums try completely different genres, so if you like one genre more, you may want to start with those albums.

Depends on what you prefer, given they often have very different sounds because all of the members wrote the songs and some of the albums will be mostly written by one member. For a sample of everything, try Gumboot soup or Omnium Gatherium, those are the albums where they put songs they didn't have an individual album for. That being said, I rather like Flight 741 (it's a collaboration album, all of the 7 members wrote 4 one), Changes (very good technically amazing jazz), Sketches of Brunswick East (also jazzy and really good), and Petrodraconic apocalypse (heavy metal with a tinge of psychedelia). To be honest though, there is standout music from each album, so just start with one that you like a song from.

If there's anyone here who's never listened to Gizz and has no idea where to start, there's tons of "beginner's guides" out there. Here's a good one though it hasn't been updated since they pulled their music from Spotify: https://www.getintogizz.com/ There's also lots of concerts on Youtube. All of the KEXP sessions are great!