Hi there,
Where is the best to store 10 different altcoins? On physical device or online?
Can I store on physical device let's say my own USB flash drive?
Myetherwallet is the best online altcoin wallet to stored your altcoins, well there are many phising link on myetherwallet, so you need to careful about it just bookmark myetherwallet site. Yes you can stored your altcoins on paper wallet/physical wallet, it's good for long term holding.