Overall, Bitcoin provides a lot of additional value over other currencies. Firstly, it provides transparency and a history for each transaction, which is not possible with most altcoins. Secondly, it has an un-stoppable network effect: the more people use or know about Bitcoin, the more valuable it becomes! Thirdly, the lower supply of bitcoins means that higher prices are required to buy them and hold them. This creates scarcity and increases demand
Bitcoin is the original cryptocurrency. That alone gives it an advantage over other cryptocurrencies. Even if there are many other cryptocurrencies which have started following the footsteps of bitcoin, none of them are as successful as bitcoin was when it was released in 2009. The reason for this is the fact that Bitcoin is safe and secure and people trust it enough to use it just like they use dollars, euros or pounds.
Simply put; bitcoin is the one and only true decentralized currency that that scale and nobody else comes even close. Of course there are decentralized ones, but the "true" decentralized requires absolutely no central authority as well, not just technically but also psychologically as well.
For example, ethereum is also decentralized and technically speaking there is no argument there, but when Vitalik says lets do this thing, everyone does that, or ltc looks decentralized but then Charlie says something or doge looks decentralized but Elon tweets about it. So, it shouldn't even have a leader like that psychologically and bitcoin doesn't have one, well the one that we had, satoshi, is nowhere to be found at least.