The superlative for costly is "most costly" or "costliest."
costliest
It should be most costly.
costlier, costliest
Tallest is the superlative.
bigger is the comparativebiggest is the superlative
smallest is the superlative. (est is the standard superlative ending._
It should be most costly.
Comparative: more costly Superlative: most costly
costlier, costliest
Comparative is more expensive. Superlative is most expensive Pork is more expensive than chicken. But Beef is the most expensive.
Costliest is an adjective. It is the superlative of costly.
Sleep is a noun and does not have a superlative. Sleepy is an adjective and the superlative is "sleepiest."
The superlative for willing would be "most willing." There is no one-word superlative.
"Bib" is a noun and, as such, does not have a superlative form.
"Studying" is not a superlative, as it is not an adjective. "Most studious" is a superlative of "studious."
most superlative is the superlative form of superlative
Tallest is the superlative.
superlative of quiet