Top Ten ?!
That's a bit optomistic.
As mentioned, the Core 3 and Xanathar's Guide are good choices. Since you are building your own setting, you might find the information in the Mordenkainen's & Volo's books beneficial, and the monsters are serviceable enough (I never picked up the Kobold Press bestiaries, so I defer to others' judgment).
It also might be worth it to look at some fan-made stuff. A lot of it is junk, but a few things stand out. The Backgrounds Omnibus adds something like 20 new backgrounds that fill-in the gaps left by the Core PHB. I think it's free... at least, I didn't spend money on it. :p
Anything else you can probably skip, unless you can find them for cheap. In fact, check ebay and similar sites. Many times you can find entire lots of books for about $15-20 apiece.