We don't know whether you want high risk or low risk investment, whether it's short term or long term but I always prefer major alts or bitcoin which means BTC or ETH. Below that you should do serious analysis and see which one matches your own idea of investing. polkadot is IMO on its peak and maybe this time around is not the right time to buy until there's dip. I'd stick with just ETH or BTC if I seek safe investment though the word safe here might be relative between you and me.