I have watched a series of videos from a famous uploader on YouTuber. I have watched a series of videos from a famous YouTuber.

either way is fine

I have recently watched a series of videos from a famous uploader on YouTube. I have recently watched a series of videos from a famous uploader on YouTube.

One of the videos that impressed me the most is clipHis videos introducing the Philippines impressed me a lot. His videos introducing the Philippines impressed me a lot.

either way is fine

One of the videos that impressed me the most is his clips introducing the Philippines. One of the videos that impressed me the most is his clips introducing the Philippines.

He said if let himhe could choose a country to live in, the most suitable country iswould be the Philippines. He said if he could choose a country to live in, the most suitable country would be the Philippines.

using "let" here is wrong

He said if let himhe had to choose a country to live in, the most suitable cwountry isld be the Philippines. He said if he had to choose a country to live in, the most suitable would be the Philippines.

The reasons are the following three: first off, there i country has beautiful scenery in the country. The reasons are the following three: first off, the country has beautiful scenery.

either way is fine. But avoiding "there is" whenever possible is usually clearer

The reasons are the following three: first off, there is beautiful scenery in the country.for the beautiful scenery, the cost of living, and the fact that most people speak English. The reasons are for the beautiful scenery, the cost of living, and the fact that most people speak English.

Secondly, the stuffprices in the Philippines isare very cheap. Secondly, the prices in the Philippines are very cheap.

Secondly, the stuff in the Philippines is very cheap.

Third, people in the Philippines all speak English.

I have never personally been to the Philippines, so ..... I have never personally been to the Philippines, so .....

I didon't know if he was telling the truth. I don't know if he was telling the truth.

or: While watching the video, I didn't know if he was telling the truth.

.... I didon't know if he wais telling the truth. .... I don't know if he is telling the truth.

Anyway, if I had time, I would go there to verify the mattersee for myself. Anyway, if I had time, I would go there to see for myself.

"verify" sounds too formal in this context

Anyway, if I had time, I would go there to verify the matter.myself to see if I agree with him. Anyway, if I had time, I would go myself to see if I agree with him.

to say that you're going to verify someone implies that you doubt they are telling the truth. In fact, you are really trying to see if his opinion is similar to yours.