Well, altcoin is such a general label these days.
Techinically, it means anything besides bitcoin, so that's like thousands of different coins lol.

But if you look at the trend this year, you will see that btc has outperformed everything else by an insanely large margin.
Look at the past trading history of btc.
It always benefits the most, especially in bull runs.
Sure, temporarily there might be something that outperforms it.
But you have to realize that in order to cash out, you have to go through btc.
And it has always gone above and beyond.