I think this is not easy. At least in the near future. The most important competitor of bitcoin is of course Ethereum. But Ethereum and other altcoins are not popular for the ordinary people. On the other hand, relatively more people are aware of bitcoin, follow it and even invest some money in it. One day I am sure one coin will replace bitcoin but the real question is when?
Agree. The altcoin which rivals Bitcoin itself is Ethereum as it is the second most largest cryptocurrency by market cap. While we cannot predict what will happen in the future with it, all the latest development and business interest into Ethereum, will definitely make it a strong competitor against the pioneer cryptocurrency.
However, all of this is up to debate, since there could exist a possibility of another coin taking over Bitcoin's place as top cryptocurrency by market cap. Bitcoin Cash had a nice uptrend in price lately, and it is currently more profitable to mine than Bitcoin itself. If it manages to gain traction from businesses and individuals alike, then it may overcome the pioneer cryptocurrency as the top coin in market cap.
Bigger blocks means more transaction throughput, and if Bitcoin Cash can deliver this, plus other advantages such as lower fees, then it may become the top cryptocurrency of choice between many crypto enthusiasts and ordinary people alike. The only downside is that bigger blocks may further decrease decentralization as the blockchain size grows at a faster rate than the original Bitcoin. But other than that, if BCH becomes successful, then it will be a major headache for Bitcoin Core.
Nevertheless, the possibilities here in crypto land are endless, and some coins can experience massive increases in price and market cap, while others get left behind. It is up to you, to make the right decision by choosing a cryptocurrency which will remain in the scene for many years to come with its active development, and popularity. Just my opinion