If you just want to keep investing on the cryptos every month without cashing out in the near future then it will be the bitcoin for your preference or you can invest on the altcoins and do short term trading with it to make your profits in small intervals but it needs your effort also.
Apart from that anyone can also try staking cryptocurrencies and get dual benefits as the price appreciates. You can also try using the money to buy cryptocurrencies like NEO, KuCoin and the rest of them that pays you dividends. There are a lot of them but no need to go further cause you can easily find them. Then apart from all these things, some people, especially those that dont want to invest money would prefer to do micro tasking.