you are seriously missing out. Technical Ecstasy and Never Say Die always get a bad wrap, and rightly so, however, there are a few good songs on each album, so if you can find them used for cheap, grab em. NSD is better than TE.
Heaven and Hell and Mob Rules are awesome. Get both. H&H is the better of the two and rivals some of the first six Ozzy albums. Get both of these before anything else.
Born Again is love it or hate it. THe only one with Ian Gillan, and I LOVE IT. "Zero the Hero" rules. It can be a task to find it, but it's worth it.
Seventh Star was an Iommi solo album marketed as Sabbath and it's pretty weak tbh. it can be difficult to find and is only for completists. its more hard-rock-cum-glam than metal.
Eternal Idol is a solid release. and started the Tony Martin era. it's not the best or worst. But Headless Cross is great and probably the best of the Tony Martin albums, though Tyr is very good and the closest Sabbath comes to being a viking metal band.
Dehumanizer bring Dio back to the fold and many hate this one, but it's got some rocking tunes. you can probably find this one for a few bucks.
Cross Purposes brings back Tony Martin and I honestly think this is a fantastic release. it does a good job of bringing sabbath into a more 90s sounding style while still being sabbath.
ive never heard Forbidden, because i dont want to hear sabbath with Ice-T, and according to reviews and critics, this is the nadir of their existence.
in short, get them all, but if you need to narrow it down, get Heaven and Hell, Mob Rules, and Headless Cross. And Born Again if you feel risky.