A few of the comparatives and superlatives in English do not follow the usual pattern. Here is a list of common exceptions.
Positive | Comparative | Superlative |
bad | worse | worst |
badly | worse | worst |
far(distance) | farther | farthest |
far(extent) | further | furthest |
good | better | best |
ill | worse | worst |
late | later | latest or last |
less | lesser | least |
little(amount) | less | least |
many | more | most |
much | more | most |
well | better | best |
The comparisons for well apply to both the adjective meaning "healthy" and the adverb meaning "in a good manner."
For more on how to use some of these see the Common Mistakes section on good/well and bad/badly. Also see Common Mistakes section for the difference between further and farther and between littlest and least.