Personally I think it is quite likely that BTC will remain 'the big one', at least in 2018. It is the safest store of value of all the crypto.
Next to that, I think we will experience a lot of projects not delivering upon their roadmap they set out in 2017, especially the ambitious roadmaps of a lot of projects that did an ICO in the previous 6 months. This might lead to a good clean-up of bad projects, but will bring a negative sentiment with it about alts with its decreasing effect on price.
Maybe from 2019 onwards we'll see BTC no longer at position 1, but still top 10. Some scalable platform coins might take over. Who will tell?