The greatest compliment one can bestow a game is that is starts out fun and stays that way. Gauntlet is a such a game.
Four player cooperative hack-and-slash didn't exist before Gauntlet. Arcade-style action games with classes of varying abilities and attributes didn't exist either. Gauntlet was a pioneering title in many ways, but its greatness is best appreciated in the fact that is was really fun to play.
Gauntlet is basically an endless sequence of labrynths, each bursting with foes, potions, various power ups, treasure chests, traps, and teleportals. The cooperative play is further enhanced by players ability to hurt themselves or the team, by shooting potions or food, stealing loot, or even shooting other players - which can stun or hurt other players on certain levels.
Like most classic arcade games, it is easy for anyone to grab a set of controls and start having a good time. Back in the day, Mom and Dad used to escape out to the golf course for a few hours and leave older brother (the warrior), younger sister (the valkyrie), and myself (the wizard) with a twenty-spot to entertain ourselves in the arcade. All three of us loved playing Gauntlet together - and there has never been a game previous or since that could ever claim that achievement. Even today, when old buddies stop by, I fire up the emulator and we run a few Gauntlets.
Definately one of the best ever.