I guess it is best to buy bitcoins when its value goes down since its high time to buy and hold them because eventually it wil rise. Although, it is also recommended to diversify your coins so if you are going to buy some altcoins instead of btc, i guess its best to invest in eth and ark. Avt is also a good alt to invest in since its project is quite interesting and a lot of investors are coming in so it's quite promising.