Ashford and Simpson's biggest hit, both critically and commercially, was "Solid," which was released in 1984. It reached the top of the R&B charts and peaked at #12 on the pop charts in the United States, while reaching #3 in the UK.

Here's why "Solid" is considered their biggest hit:

Commercial Success: "Solid" was their first R&B chart-topper and reached the top 12 on the pop charts in the US.

Chart Performance: It performed well in the UK, reaching #3.

Defining Sound: "Solid" is often cited as a song that defined the synthetic sound of 80s R&B.

Album: The song also gave the album it appeared on, "Solid," its name.