Bitcoin,there is no doubt in it that the price of bitcoin keeps increasing on the long term. Altcoins are good for short term investment but for long term investment, Bitcoin is the best option. What I do is buy bitcoin and hold it for long term while i trade on altcoin for short term.
If you wish to invest on bitcoin you can just buy and hold it. That will give you better price in future for sure but for altcoin investment you cannot do that because of the little value and some cheap altcoins may go back to reach the scratch value.
Now in altcoin investment, people are concentrating to invest on mining side in bitcoin trading goes with the more profit with fast fluctuation in the price index.