What are the major aircraft systems (LRUs) using the ARINC429 Protocol?
On modern aircraft all equipments that needs to communicate to each other in binary format may use ARINC 429 protocol. For instance all fly by wires computers on Airbus 320, 330, 340 use arinc 429 protocol.

Nowadays other protocols do exist too such as arinc 629 protocol or fiber optic protocols etc.

As a summary all aircraft systems that need to transmit and receive binary data may use 429 protocol, but other digital protocols are possible.

However a standardized LRU that doesn’t use ARINC 429 protocol is the FDR ( flight data recorder), because the data that it receives is already sequentially ordered, the label is not transmitted with the data value

On the following website starting page 43 you find all the possible equipment that may use A429

http://read.pudn.com/downloads111/ebook/462196/429P1-17_Errata1.pdf

Also in the same document starting page 13 you have the list of the parameters that may be transmitted on A429

Hope this added information is helpful