This game pretty much single handily invented the Real-Time Strategy genre on PC. If it wasn't for Dune 2 game series like Command and Conquer, Warcraft and Starcraft would have never existed.
If a game is still entertaining 20+ years after being released it is a classic in my mind. Some games released today rely so heavily on graphics that they have zero replay value and just aren't that fun to play... but hey they look amazing!!! Dune 2 doesn't have the advanced AI or controls of current RTS games yet it still plays amazingly well for a game released in 1992.