I am not quite sure, which altcoin to choose. It is not simple - hundreds of various altcoins.
There are many multi-coin wallets or altcoins wallets out there I know some of the wallets you can use for multi-coins or many supported altcoins.
Check this list of wallets.
> Coinomi wallet - they recently released the Desktop version and I'm using their mobile version for a long time without a problem.
> Zelcore wallet - they have limited supported altcoin but almost all major altcoin is there
These two wallets that I know you can use for multi coins but you don't be able to transfer or send offline. However, if you want to send bitcoin to someone offline much better fill the wallet first the amount that you would like to send to someone then give the private key to him offline.