While it took him until 2004 to release a greatest hits compilation, his solo career kicked off in 1968 after the break up of Buffalo Springfield, Neil Young's Greatest Hits was worth the wait.  For the true Neil Young fan, you likely have all of these songs spread across many CDs so worth pulling them together in a single place, but for the uninitiated this is a great gateway into Neil Young's career. 

Focusing primarily on his early work which established him as the Neil we have all come to love, it doesn't cover anything in the last ~15 years.  That being said, classics like Down by the River, Cowgirl in the Sand, After the Gold Rush, Southern Man, Heart of Gold, Harvest Moon, etc. are the core of Neil's catalog and having them in a single disc means it's just keeps getting played over and over and over again.  After all these years, Neil's still rocking in the free world.