Lo que menos me gusta es madrugar en invierno.

Here lo que means what or the thing that.

So:

• Lo que menos me gusta = What I like least / The thing I like least

This lo is neuter, so it does not refer to a masculine noun. It refers to an idea or thing in a general way.

Menos is used to express least in this kind of structure.

• lo que más me gusta = what I like most
• lo que menos me gusta = what I like least

So menos is not saying something is worse; it is saying it is the thing you like in the smallest amount.

Because gustar works differently from English to like.

In Spanish:

• the person who feels the liking is an indirect object: me = to me
• the thing liked is the grammatical subject

So:

• me gusta el café literally = coffee is pleasing to me
• me gusta madrugar literally = getting up early is pleasing to me

In Lo que menos me gusta, the subject is lo que = what/the thing that, so gusta is singular.

Because the subject is treated as a single thing:

• lo que menos me gusta = the thing that I like least

That whole idea is singular, so Spanish uses gusta.

Compare:

• Me gusta el libro = I like the book
• Me gustan los libros = I like books

Here, it is not a plural noun, so gusta is correct.

Because the sentence is equating two things:

• Lo que menos me gusta = one idea
• madrugar en invierno = the thing being identified as that idea

So es means is:

• What I like least is getting up early in winter

Also, madrugar is an infinitive, and infinitives can act like nouns in Spanish, just as getting up early does in English.

Because this is about identity/definition, not a temporary state or location.

• es is used for saying what something is
• está is used for location, condition, or temporary state

Here the sentence means:

• The thing I like least is...

So ser is the correct verb: es.

Madrugar means to get up very early or to wake up early, usually with the idea that it is especially early.

It is a very common word in Spanish, especially in Spain.

It is a bit stronger or more specific than just:

• levantarse temprano = to get up early

So madrugar often suggests an early start that may feel unpleasant or demanding.

Because after en, Spanish often uses the season without an article when speaking generally.

So:

• en invierno = in winter
• en verano = in summer

This is very natural when talking about a season in general.

You may see the article in other contexts, but here en invierno is the normal choice.

Most naturally, it goes with madrugar:

• madrugar en invierno = getting up early in winter

So the meaning is:

• The thing I like least is getting up early during winter

It is not just saying in winter, what I like least is... although in practice the English translation can sound similar.

Yes. That is also correct.

Both sentences are grammatical:

• Lo que menos me gusta es madrugar en invierno
• Madrugar en invierno es lo que menos me gusta

The difference is mainly emphasis:

• Lo que menos me gusta... starts with what I like least
• Madrugar en invierno... starts with the specific activity

The original version sounds a bit more like a neutral statement of preference.

Yes, but it means something slightly different.

• Lo que menos me gusta es madrugar en invierno = The thing I like least is getting up early in winter.
• Lo que menos me gusta del invierno es madrugar = The thing I like least about winter is getting up early.

The second one specifically says you are talking about winter and what you dislike about it. The original is a bit broader and focuses on the activity madrugar en invierno as a whole.