At my job, no one is better than I, Day and night, keeping a watchful eye, My boss is pleased when they reap their yield, Yet I stay outstanding in my field.

The first man says he has me, he uses me right; The second only thinks he does, he's not a great mind; The dumbest is the third one, who says he truly doesn't; But if he spoke the truth, he would be six feet under.

A slang for breast, I can hold many liquids, I can also be used to make music, or be found in old cartoons with three x's

Don’t look so glum, For you’re a peach, And I’m just a. . .

In a rush, you are of course! Just don't put me before the horse.