harmful
more harmful, most harmful
"Higher" is the comparative of high. The superlative is highest.
Comparative: lazier Superlative: laziest
'Next' is a superlative; it means 'nearest'. The positive is therefore 'near' and the comparative is 'nearer'.
last
Comparative: easier Superlative: easiest
"More harmful" and "most harmful." We would not use the -er and -est suffixes on the -ful which is already a suffix.
finer, finest
older, oldest
stronger and strongest
busier, and busiest
The comparative form of rude is ruder, and the superlative form is rudest. It is common to form the comparative and superlative of short words like this by adding -er and -est.