Learn Swedish: Den här möjligheten är nog bäst. - This opportunity is probably the best.

It is built from the adjective möjlig (possible) and the suffix -het (which works like "-ity" or "-ness" in English). A great rule to remember is that all Swedish nouns ending in -het are en-words. Because of this, it takes the definite ending -en to become möjligheten and uses den här instead of det här.

Why does the sentence use den här möjligheten with the -en ending, instead of just den här möjlighet?

This is a reminder of the Swedish "double definite" rule. When you point something out using den här (this) or den där (that), the noun that follows must also be in its definite form. Since möjlighet is an en-word, we add -en.

Doesn't nog usually mean 'enough'? Why is it translated as 'probably' here?

You are right that nog means "enough" when it comes directly after an adjective or adverb (like bra nog for "good enough"). However, when it is used as a standalone little word in the middle of a sentence like this, it changes meaning to "probably", "likely", or "I reckon".

Since we are talking about a specific, definite opportunity, why isn't it bästa?