When comparing two things or people, use -er or more.
Brad is the taller of the twins, but Chad is more handsome.
When comparing more that two things or people, use -est or most.
Of the four Pearl Cleage plays we read in class, Flying West has the most appealing characters.

Avoid the following:

DOUBLE COMPARISONS

Do not use both an -er/-est ending with the words more/most: friendliest, NOT most friendliest.

INCORRECT COMPARISONS

Use a dictionary when you are uncertain. friendlier, not more friendly; lonelier, not more lonely.

ILLOGICAL COMPARISONS

CHANGE The interest at a loan company is higher than a bank
TO higher than a bank’s
OR higher than that of a bank
OR higher than the interest at a bank.

AMBIGUOUS COMPARISONS

Make sure that the meaning of the sentence is clear to the reader.
CHANGE Matt likes pizza better than Cheryl
TO better than Cheryl does
OR better than he likes Cheryl.

INCOMPLETE COMPARISONS

CHANGE Television advertising is more effective
TO more effective than magazine advertising.

CHANGE Television is as effective if not more so than radio advertising (as is missing)
TO Television advertising is as effective as radio advertising.

CHANGE Los Angeles has more freeway traffic than any city in the U.S. (other is missing)
TO than any other city in the U.S.