It may not be pretty, but Half Life has maintained its place at the top because of fun factor and nostalgia.
Yes, if you didn't get it from my introduction this is a wonderful game. From top to bottom the fun never stops. In Half Life you play as Gordon Freeman, an employee at the Black Mesa Research Facility in New Mexico. You were assigned to take part in an experiment. Unfortunately for you however that experiment goes horribly wrong and the facility is destroyed. You, a former useless employee is now the only hope for everyone that managed to stay alive. Gun down mutants as you try to find the secrets and escape from the facility!
The single player portion of the game is a great experience but the multiplayer is something special. Play a traditional deathmatch game with all kinds of cool guns. Choose from an assortment of player models and duke it out with others. Modders have been able to add teamplay to Half Life as well. Unforunately for Half Life, the graphics have not aged well. All of the characters are very blocky and are an eyesore to look at. However this was one of the first games to have mouths open and close while people were talking so that was a big hit. Back in the day the graphics were great though.
The sound on the other hand is still at the expected level of sounds today. From extremely annoying bugs to shotgun shots to alarms, all of Half Life's sounds are of the utmost quality!
All in all, if this did not convince you this is a game worth playing, nothing will. However if you are like most gamers you will love this game!