The safest way to keep BTC is Bitcore or handy wallets like Trezor and Ledger. Some of them might also support ETH and some coins like LTC and Monero (I call them Old-school coins), you won't find safe multicurrency wallets supporting SIA. And Sia's desktop wallet isn't good also, there are a lot of people on their forum talking about their money losses transferred to this desktop version (same was for me, I lost 2000 and haven't received it yet because it generates new adress every new transaction). So my advice is to hold it not more than 5000 SIA on Poloniex.
Good alternative might be NEM with their very convenient mobile wallet.