Bitcoin is the safest crypto investment because simply this coin is right now the most successful cryptocurrency, and it have the highest potential and trading volume among all cryptocurrencies. Bitcoin is also more stable than any other altcoin, because the demand for BTC is increasing daily almost.
Since this is the first crypto was being made, theres no doubt to be lead among those coin and most popular than the others. As long as your private k. Will keep store in your safe storage, it will remain safe. You will not be worried about the security of bitcoin system because it has really strong and i dont ever heard about that issue.