Ryzen CPUs
Socket AM4

The links above summarize the specs and costs for Ryzen CPUs, the specs for Socket AM4 chipsets.

ASUS offers 14 different AM4 motherboards:
A320 ... PRIME A320M-A, PRIME A320M-C, PRIME A320M-E
B350 ... PRIME B350M-A, PRIME B350M-E, PRIME B350-PLUS, ROG STRIX B350-F GAMING, TUF B350M-PLUS GAMING
X370 ... PRIME X370-A, PRIME X370-PRO, ROG STRIX X370-F GAMING, ROG CROSSHAIR VI HERO (WIFI, non-WIFI), ROG CROSSHAIR VI EXTREME

ASUS offers 3 different TR4 motherboards:
X399 ... PRIME X399-A, ROG STRIX X399-E GAMING, ROG ZENITH EXTREME

The usual two questions for anyone buying a computer:
1) What do you plan to use your computer for?
2) How much are you willing to spend?

My rough guideline is:
- AM4 motherboard ... PRIME A320M-A is basic entry-level, PRIME B350-PLUS is better entry-level, PRIME X370-PRO is best entry-level, either STRIX is a great gaming choice, either CROSSHAIR is a top-end performer.
- AM4 Ryzen CPU ... R3-1200/-1300X and R5-1400 are cheapest but really too gutless, R5-1500X/-1600/-1600X are the sweet spot, R7-1700 is a poor choice, R7-1700X/-1800X are the fastest and most costly.
- TR4 Ryzen CPU (Threadripper) is highest cost, highest performance, highest technical complexity. X399 is not for everyone.

AMD promised to launch slightly better "PRO" versions of these Ryzen CPUs by September 2017, but now they're already late. You might be able to get something newer and slightly better (or get existing things slightly cheaper) if you're willing to wait a month.

If you plan on building an extreme machine or doing any kind of overclocking with AMD parts then you'll need to select your parts (especially DDR4 memory) carefully and be prepared to spend time reading online guides.