harmful
more harmful, most harmful
"Higher" is the comparative of high. The superlative is highest.
lazier and laziest
'Next' is a superlative; it means 'nearest'. The positive is therefore 'near' and the comparative is 'nearer'.
last
easier and easiest
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.
busier, and busiest
stronger and strongest
older, oldest
finer, finest
"More harmful" and "most harmful." We would not use the -er and -est suffixes on the -ful which is already a suffix.