I stored my altcoins in MyEtherDelta however there are news  that MEW was compromised.
Anyway I have just read in reddit that it's not true.
https://www.google.com.ph/amp/s/amp.reddit.com/r/ethereum/comments/7p6c8v/is_mew_website_being_hacked/
So it is still safe to store your altcoins erc20 in MyEtherWallet. Just be careful though.