Found a better package elsewhere, if they like you they will match or exceed it.
Travel to and from work is excessive and you will be losing your transportation.
You do not feel that you are progressing at your current workplace in a manner that reflects the effort you put in.
You have a terminal illness.
You're moving out to another town/city/suburb.
You'd like to focus on other aspects of your life at the present time, study/family.
You feel that what you did in the toilets with one of the other employees will put a strain on your work relationship.
You wish the company the best of luck but no longer believe that you share the same enthusiasm for it's success and rather than become an unproductive member of the team wish to tender your resignation.
Whatever you do wish the company the best of luck, you don't have to tell them it's because they're all incompetent so they will need it just wish them luck and all the best
Don't go whingeing about what was wrong, you had plenty of time while working there to bring these things up.
Edit: Basically it's like trying to break up a relationship nicely: "It's not you it's me"
Unless they really cheesed you off but then you might not get a reference later on so it's up to you how nice you will be.