There are subtle differences when using a superlative for an idiomatic group or when using it simply with plural. This can nicely be shown with the examples you gave.
Superlative and plural
It was the darkest of nights. - Es war die dunkelste aller Nächte.
Note that we can use "aller" here to put emphasis on the fact that a variable number of many nights are judged here. Similar examples would be
- Fritz ist der beste aller Schüler [pl.].
- Die schönste aller Blumen ist die Rose.
We can of course replace "aller" (meaning the sum of all) by any other expression to precise the quantity:
Das dickste der fünf Bücher, die ich gelesen hatte war...
Die hübscheste meiner Töchter ist die Susi.
Das beste Deiner Programme ist...
Only if the quantity is invariable we may also use the simple plural:
In diesem Strauß ist die schönste der Blumen eine Rose.
This is however only rarely done as then using singular is preferred:
Die schönste Blume in diesem Strauß ist eine Rose.
Fritz ist der beste Schüler [sing.].
Superlative and a named group
Lake Michigan is the largest of the Great Lakes.
In this case we are dealing with the fixed name Great Lakes where the quantity is invariable and thus we translate
Lake Michigan ist der größte der Großen Seen.
Note the difference when we say
Lake Michigan ist der größte aller großen Seen.
In the latter case the superlative relates not to the Great Lakes but to all great lakes in the world.