Exodus has Bonded By Blood which is quite possibly the greatest thrash album of all time. But the rest of their catalogue is... not very good.
Overkill is a lot more consistent. Horrorscope, The Years of Decay, Feel the Fire, Ironbound, The Electric Age, Taking over... it's all good. Well, honestly, their earlier albums up until Horrorscope are all good. Then they have this shitty mid-period where they did shitty groove bro metal and that can be safely ignored. Then they came back to thrash with a vengeance with Ironbound, and have kinda just recycled that sound ever since but at least it's still pretty good, so... cool, I guess! :)
Edit: ah yeah, ^ the holy trinity of early thrash, still tinged with that NWOBHM sound; Feel the Fire, Kill 'em All, and Show No Mercy. Love those albums, definitely my favourites by Metallica and Slayer, and definitely up there for Overkill too.