I think bitcoin will still dominate this years and probably until the first quarter of 2019, and many users invest in bitcoin, it is good to invest one's money in bitcoin, or if you just have that high funds, you can invest 70% on bitcoin and 30% on ETH.
Actually bitcoin always take the leads of all cryptocurrency investment, but looking for best investment this year still bitcoin is the best as this coin is the father of all cryptocurrency. Or you can look any altcoins which also have great potential to grow before this year ends. It depends upon of what your best choice among all altcoins.