Half-Life is easily the best game ever made, no doubt about it.  The graphics are a little out dated but thats okay the gameplay more than makes up for it.  It isn't perfectly realistic but it comes very close to it.  For one, you can carry 8 weapons at a time but thats the case it most shooters.  And head shots don't kill.  But besides that the game is very realistic.  Guns are in logical places not just lying there as with health, health power-ups aren't just sitting there, instead you get health at refill pump which also are in very logical places.  Now as the plot goes, the most brilliant part in the game.  You are a scientist who works at Black Mesa Reseach Facility and are working on a very big project which goes very wrong, warping aliens to the lab.  Now the lab is infested with deadly aliens and you are the only one who can save you fellow scientists.  Wanna find out what happens?  THEN BUY THE GAME!!!