Lo mejor del viaje fue ver el mar al atardecer.

Lo is the neuter article in Spanish. It is often used before an adjective to mean the ... thing, what is ..., or the ... part.

So:

• lo mejor = the best thing / the best part
• el mejor = the best one (masculine person or thing)

In this sentence, lo mejor del viaje means the best part of the trip.

Del is just a contraction of de + el.

• de = of/from
• el viaje = the trip
• de el viaje becomes del viaje

This contraction is mandatory in standard Spanish whenever de is followed by el.

So:

• lo mejor del viaje = the best part of the trip

Fue is the preterite form of ser for he/she/it was.

Here it links the subject lo mejor del viaje with what that best part was:

• Lo mejor del viaje fue ver el mar al atardecer.
• The best part of the trip was seeing the sea at sunset.

Spanish often uses ser in this kind of definition or identification sentence:

• Lo difícil fue empezar.
• The difficult part was starting.

This is a very common question because both can translate as was.

Here, fue sounds natural because the speaker is treating the trip as a completed whole and identifying one thing as the best part of it.

• fue = completed, viewed as a whole
• era = more descriptive, background-like, less bounded

So:

• Lo mejor del viaje fue ver el mar al atardecer.
  = a natural summary of a finished trip

You might hear era in some contexts, but it usually sounds more like description or reflection rather than a completed summary.

In Spanish, an infinitive can act like a noun phrase.

So ver el mar al atardecer works as seeing the sea at sunset.

This is very common after ser:

• Lo mejor fue descansar.

• The best thing was resting.

• Lo peor fue esperar.
• The worst thing was waiting.

So the structure is:

• Lo mejor del viaje = subject
• fue = linking verb
• ver el mar al atardecer = what the best part was

Spanish usually uses the article more often than English does.

Here, el mar is the natural way to say the sea. In English, you might sometimes say see the sea or just see the ocean/sea, but in Spanish the article is normally kept.

So:

• ver el mar = to see the sea

Also, mar is normally masculine in everyday standard Spanish:

• el mar

You may sometimes see la mar, but that is more poetic, regional, or stylistic.

The personal a is generally used before a specific human being (and sometimes a pet or personified animal) when that person is the direct object.

Examples:

• Veo a María.

• I see María.

• Veo el mar.
• I see the sea.

Since el mar is not a person, there is no personal a here.

Al atardecer means at sunset, at dusk, or toward evening, depending on context.

It comes from:

• a + el = al
• atardecer = sunset / dusk / nightfall, or also the verb to get dark / to grow late

In this sentence, al atardecer is a time expression:

• ver el mar al atardecer
• to see the sea at sunset

It does not mean movement toward something here. It simply tells you when.

Yes. That version is completely correct.

The difference is mainly focus:

• Lo mejor del viaje fue ver el mar al atardecer.
  This starts with the best part of the trip and then explains what it was.

• Ver el mar al atardecer fue lo mejor del viaje.
  This starts with seeing the sea at sunset and then says that it was the best part.

Both are natural. Spanish allows this kind of flexibility more easily than English.

No subject pronoun is needed here.

The grammatical subject is the whole phrase:

• Lo mejor del viaje

That is what fue agrees with. Spanish usually does not need an extra subject pronoun unless there is a reason for emphasis or contrast.

So the sentence is already complete as it stands.

A near-literal breakdown is:

• Lo mejor = the best thing / the best part
• del viaje = of the trip
• fue = was
• ver el mar = to see the sea
• al atardecer = at sunset

So literally:

• The best part of the trip was to see the sea at sunset

More natural English would usually be:

• The best part of the trip was seeing the sea at sunset.

Yes. That can be confusing for learners.

As a noun:

• el atardecer = sunset / dusk

As a verb:

• atardecer = to get dark / to become evening

In al atardecer, it is best understood as a fixed time expression meaning at dusk / at sunset. Learners usually do not need to analyze it too deeply every time; it functions like a set phrase.