I've heard all 100+ entries in Zappa's discog. Most is mediocre/bad even if you're a hardcore fan like me so stick with me.
Imo, there's four main "eras" to Zappa:
First I'd ask you what you thought of those albums you heard. What did you like/dislike about them? Which did you prefer? Joe's Garage falls under the blanket of "Comedy Rock" while Over-nite Sensation is the "Jazz Fusion Era".
For every era there's one super important starting point, and where I'd recommend getting started on him.
Experimental Rock: Freak Out
Jazz-Fusion: Hot Rats
Comedy Rock: Apostrophe
I'd say decide what kind of music you'd want most out of the guy. If you like his experimental stuff I'd say go forward through his discog starting with Freak Out (skipping Cruising With Ruben & The Jets and maybe Lumpy Gravy) and stopping when you quit enjoying his albums.
If you wanted Jazz Rock, I'd say hear Hot Rats then go backwards through his Mothers discog and hear Waka/Jawaka and Sleep Dirt (often called Hot Rats 1 & 2). His live albums like Roxy & Elsewhere are essential as well.
As for Comedy Rock (my least favorite of his stuff) this encompasses the majority of his work, so I'd say just choose whatever from the latter half of his career not including his posthumous stuff. Apostrophe is a great starting place for this era because it was his first album blurring the era between Jazz-Fusion and Comedy Rock.
His reissue/posthumous stuff is only worth getting into if you've already listened to most of his stuff. Often they're poorly recorded, worse versions of what's already been released, or just bad material he never released for a REASON (I don't care for the Zappa family trust much lol).
Zappas discog is long and most stuff after the mid-70s isn't amazing, but his good stuff is very rewarding. A good example of this is RDNZL, one of his best songs smushed on the back of a pretty mediocre album "Studio Tan".