You cannot judge by a one-month situation only. Yes, in May, Bitcoin showed more power than altcoins, but it seems to be natural - the leader must do it. Further, altcoins will be growing fast as well. I bet on ETH and Bitcoin Cash.
one way or another ethereum occupies the lions share of the cryptocurrency market. If you still take into account all successful ico companies that used the erc20 platform, then everyone is much more likely to be in a leading position among all altcoin.