Which country is better to get the visitor Visa to the USA?

You should apply at the US Consulate in the country where you are normally resident.

The country is irrelevant - you apply from wherever you're a resident as a citizen of your country.

You're a citizen of Guinea. You cannot even file a visa application with the consulate in Sierra Leone unless you are legally resident there!

Rules for visas are exactly the same for citizens of any country. Your personal experience in living, working, traveling always within the laws of the country/ies where you go matters. So what's your track record in abiding by visa regulations and/or immigration laws of every country you've ever entered? And then, what's the incidence rate for visa fraud for citizens of your country when applying for US visas?

And do you meet all requirements of whatever type of visa you are applying for? E.g. tourist visa: sufficient funds of your own for all typical tourist expenses (round-trip tickets, hotels, restaurant meals, sightseeing, local/ground transportation, etc.), and ties to your country which compel you to return home timely.

It doesn't matter where you apply from, you apply to the US visa site/US consulate as all visas are agreed or denied in the same place which is US immigration which is in the USA

apply in the country that holds your passport