Here are a few suggested phrases:

For the chronically absent:
"A man like him is hard to find."
"It seemed his career was just taking off."

For an employee with no ambition:
"He could not care less about the number of hours he had to put in."
"You would indeed be fortunate to get this person to work for you."
"He consistently achieves the low standards he sets for himself."

For an employee who is so unproductive that the job is better left unfilled:
"I can assure you that no person would be better for the job."

For an employee who is not worth further consideration as a job candidate:
"I would urge you to waste no time in making this candidate an offer of employment."
"All in all, I cannot say enough good things about this candidate or recommend him too highly."

For a stupid employee:
"There is nothing you can teach a man like him."
"I most enthusiastically recommend this candidate with no qualifications, whatsoever."

For a dishonest employee:
"His true ability was deceiving."
"He's an unbelievable worker."