I prefer summer thano winter

I prefer summer thano winter because the days are sunnier and the weather is warmer.

The only thing that winter is bestter than summer in my opinion is to sleep.

My sleep quality is better on winters.

I prefer summer thano winter

The right preposition is "to"

I prefer summer thano winter because the days are sunnier and the weather is warmer.

The adjectives should be comparative when comparing two things

The only thing that winter is bestter than summer in my opinion is to sleep.

Use better than

My sleep quality is better oin winters.

I pPrefer sSummer than wover Winter

Important words in title have capital letters. "over" or "to" not "than".

I prefer summer thano winter because the days are sunnier and the weather is warm.

"to" or "over" are used for the comparisons not "than". You could also say "the weather is warmer". i.e. a comparative form.

The only thing thaabout winter that is bestter than summer, in my opinion, is tohow well I sleep.

"about winter" is used to talk about the properties of winter e.g. temperature, day light. "better" you need the comparative form not the superlative "best". "how well I sleep" is used to refer to the quality of your sleep.

My sleep quality is better oin winters.

Preposition is "in" not "on" with respect to it being during winter. Use "winter" rather than "winters" to describe how winters as a whole.

I prefer summer thano winter

Use "prefer to" when comparing two things.

I prefer summer thano winter because the days are sunnier and the weather is warm.

The only thing that winter is best than summer in my opinion is to sleep.

For better flow;
The only thing that winter is better than summer at, in my opinion, is sleeping

I prefer summer thano winter

I prefer summer thano winter because the days are sunnier and the weather is warm.

The only thing that winter is best than summer in my opinion is toI prefer about winter is sleep.

I have largely simplified this sentence so make it easier to understand.

My sleep quality is better oin (the) winters.

We would more naturally say "my quality of sleep" rather than "my sleep quality"