I would go with ETH at the moment. It is very underpriced relative to the others, and is a great project.
BTC is obviously a good buy as well, but look at the ETH price chart relative to BTC, and it suggests that ETH is currently the better buy and should improve vs BTC in the near future.
As for EOS, I think a lot of the price is still based on hype, so it may be overpriced at present.