Cryptocurrency is not yet legal in many countries of the world
You should know that there is a difference between not being legal and not being regulated. Bitcoin is not regulated in many countries but it is still legal.
There are also specific cases in cryptocurrency world that have been banned in some countries. For example buying/selling/owning bitcoin is legal in China but creating or participating in ICOs is illegal.
The real problem is that my country is not a cryptocurrency due to which there is no system of Decentralized Exchange to buy / sell our Cryptocurrency and bring it in its own currency. That's why we have to lean towards centralized exchanges, so which is the best in such a situation, so I wanted to know your opinion.
Your first sentence makes no sense. Is cryptocurrency illegal in your country or not regulated?
If you in a hand of of countries that have actually banned bitcoin then you don't really have that many good choices since you are breaking the law after all. But if it is the later then any option is a good option.
First educate yourself about how bitcoin works, what wallet is the best and how you can secure your coins.
Then see which CEX you can use depending on where you live that is more convenient. Coinbase is a popular one that many use but they all would require KYC which is a privacy hazard.
There is also always other people in different countries that own bitcoin and are willing to trade. You can find them on the internet, this forum also has a
currency exchange board that you can find others to trade with. Keep in mind that you should always use an escrow and educate yourself about the risks.
There is also Bisq that was already introduced.