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BTC
Because it´s required to buy/trade most of all other coins.

ETH
Because it will go to the moon. :)

LTC
Fast. Cheap. Stable.

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There is no need for all the other coins and most of them will hopefully die ASAP.

I agree to the fact that many of them will not see the light after 2018. But even with Doge coin reaching $1B valuation, anything is possible :D