Heavy
heavier, the heaviest
Young
younger, the youngest
Big
bigger, the biggest
Good
better, the best
Slow
slower, the slowest
Clever
cleverer, the cleverest
Fast
faster, the fastest
Fat
fatter, the fattest
Small
smaller, the smallest
Strong
stronger, the strongest
Bad
worse, the worst
Pretty
prettier, the prettiest
Thin
thinner, the thinnest