It wasn't obvious which route would be best. It wasn't obvious which route would be the best one to take. If you use "the best", it should modify something - in the above case "one", meaning "route" [Note: almost nobody will notice if you use either one. This is one of those English grammar questions about which some of us disagree (see Eliot's answer). It just doesn't matter that much.]

1. That would be the best route. = That route would be the best. 2. Taking that route would be best. = That route would be best. Study those two sentences.

Both phrases are fine in that particular sentence, but "be best" would probably sound a bit more fluent. If the phrase starts out as "It would probably be..." then say "It would probably be best" not "the best." Example 1: "It would probably be best if we left 30 minutes early. Then we can avoid traffic." Example 2: "It would probably be best to call him tomorrow. I think he's in a lot of meetings today." Another spoken, native American-English way to reword your sentence is to say: "The best way to get there is not obvious." Similar phrases: "I'm not sure the best way to get there." "I'm not sure which route would be best." "I'm not sure which route would be the best way to get there."

I would agree with the person who said it is more fluent to say, "what would be best...". Also, when you add the article, 'the', there really should be a noun after it. So, the person who said, "What would be the best way..." is following this rule because 'way' is the noun that relates to the article.

its would definitely be "the best"... The definite article "the" is used before superlative adjectives.Here in the sentence "best" is superlative.(good-better-best)