When you compare within a same thing, don't you add "the" like the following? Is it true?
ex)The road is widest around here.
or The river is deepest here.
The road is widest around here.
The river is deepest here. Both of these are correct. Is that what you are asking?
If you rephrase them like this: "This is the widest part of the road", the article is necessary before "widest".