I vote BTC. It's because BTC has more stable value than Alt and also has many advantage than Alt that make BTC suitable for long term investing. i agree that BTC has slow transaction speed and high fee. But it's because many people do transaction with BTC everyday. And If ETH or another ALT became the biggest Coin or King of crypto, they will have this problem.
For the long term, of course bitcoin is more interesting than altcoins, but if we aim to get big profit then altcoins is the most appropriate thing, this is because altcoins price increase can reach hundreds of percent per day.