The best part of my outfit is Ymy T-shirt.

I corrected “Y-shirt” to “T-shirt” because that is the correct term. Adding “my” before “T-shirt” makes the sentence sound more natural and shows ownership, which is typical when talking about your own clothes.

I bought it to summer season few years agoa few years ago for the summer season, and it is made bywith special ingredients with which I canmaterials that help me feel cooler.

The phrase “to summer season few years ago” is grammatically incorrect and unnatural in English. Saying “a few years ago for the summer season” puts the time phrase in the right order and uses “for” correctly to explain the purpose of the purchase. Also, clothes are made from “materials,” not “ingredients,” so “made with special materials” sounds natural. The phrase “that help me feel cooler” is simpler and easier to understand than “with which I can feel cooler,” making the sentence flow more smoothly and sound more conversational.

It has been unbelievably hot these days, so this YT-shirt really saves my life.

I kept “T-shirt” consistent in this sentence, and added “really” to make the sentence sound more natural and to emphasize how much the T-shirt helps you during hot days.

The best part of my outfit is Ymy T-shirt.

I bought it tofor the summer seasona few years ago, and it is made byout of special ingredients with which I canmaterials that make me feel cooler.

It has been unbelievably hot these days, so this YT-shirt saves my life.

The best part of my outfit is Y-the shirt.

I bought it toduring summer seasona few years ago, and it is made byof special ingredients with whichso I can feel cooler.

"made by A" -> A is the person or company that made an item
"made of B" -> B is a material used to make an item

It has been unbelievably hot these dayslately, so this Y-shirt saves my life.

these days is a bit of a wider time period than lately. Assuming you're talking about the last few days/weeks, I think lately is better.

The best part of my outfit is Y-the dress shirt.

By "Y-shirt", are you referring to ワイシャツ? In English, they are known as dress shirts. Most English speakers will not understand what a Y-shirt is.

I bought it toduring (the) summer season a few years ago, and it is made bywith special ingredients with which I canmaterials which feel cooler.

(1) The phrase is "made with (something)". "Made by" is used when referring to the creator, such as in the phrase "this shirt was made by Uniqlo" or "this car was made by machines".
(2) "Materials" might be more suitable here. "Ingredients" is typically used in the context of food (though it can be used in other situations too).
(3) "With which" here strikes me as a little unnatural. "Which feel cooler" will suffice.

It has been unbelievably hot these days, so this Y-dress shirt saves my life.

What is the best part of your outfit today?

If you want this as a proper title: What Is the Best Part of Your Outfit Today?

The best part of my outfit is my Y-shirt.

I bought it toduring the summer season few years ago, and it is made byof special ingredients with which I canhich feel cooler.

I simplified the ending here for something more natural.

Things are made OF something: the bowl is made of wood
Things are made BY people (or companies): the bowl was made by Mary