the most valuable coin to buy at this time is bitcoin because the price of bitcoin which is still very cheap will give you an advantage when you buy it at a cheap price like now and will provide benefits for you when prices have started to bounce up again.
and the worse the altcoins in your opinion 
they are also very much cheaper and can make a profit much more than Bitcoin. I am sure that most of the top Altcoins will continue to develop next year and this allows us to invest in them now while they are cheap.