The comparatives and superlatives of these adjectives usually have three or more syllables and are formed by adding more and most.

Positive Comparative Superlative
ancient more ancient most ancient
beautiful more beautiful most beautiful
brilliant more brilliant most brilliant
careful more careful most careful
careless more careless most careless
cheerful more cheerful most cheerful
comfortable more comfortable most comfortable
dangerous more dangerous most dangerous
delightful more delightful most delightful
difficult more difficult most difficult
enjoyable more enjoyable most enjoyable
expensive more expensive most expensive
foolish more foolish most foolish
forgetful more forgetful most forgetful
frightening more frightening most frightening
generous more generous most generous
handsome more handsome most handsome
helpful more helpful most helpful
ignorant more ignorant most ignorant
important more important most important
intelligent more intelligent most intelligent
interesting more interesting most interesting
pleasant more pleasant most pleasant
powerful more powerful most powerful
prosperous more prosperous most prosperous
sensible more sensible most sensible
terrible more terrible most terrible
thoughtful more thoughtful most thoughtful
unusual more unusual most unusual
useful more useful most useful
valuable more valuable most valuable
wonderful more wonderful most wonderful