1> which is the fastest, most secured and decentralised blockchain already working ( i.e mainnet already running)
1. fastest and most secure?
Bitcoin. Because it is the most secure blockchain out there by far. You can have higher speed that some other chains promise, but if someone double spend your coins tomorrow, what good does this speed bring?
If we are talking about security, no doubt Bitcoin is the best in my opinion. But the fastest coin? no, it is not.
let's not talk about XLM and many others, Bitcoin transaction speed is lower compared to ETH. Both of them until now, can not exceed 20 transactions per second. How come, it is the fastest coin in our cryptosphere, if something else can do 100 transactions per second or even more than that?
about double spending, every PoW project has a probability to get double spending. That means Bitcoin has a probability to get it too.