Aquí es que he utilizado, es bueno, aunque es un ser pagada ahora (pero se puede pedir un juicio)
https://weibomiaopai.com/api-documentation/
Ejemplo del uso:
https://uploadbeta.com/api/video/?cached&video=https://www.youtube.com/watch?v=Sjs1DAmcwB8
retorno s:
{"host":"youtube.com","server":"uploadbeta.com","url":"https:\/\/redirector.googlevideo.com\/videoplayback?lmt=1427909364205998&signature=A420DE8B19EC02EAEB0C8B3F6AF643054D5B047C.9273940FFAB687A439EAACEE23925C5CC7553B1D&ipbits=0&mime=video%2Fmp4&itag=18&key=yt6&ms=au&mt=1497991169&mv=u&dur=254.328&ratebypass=yes&source=youtube&clen=23224917&mn=sn-ab5l6ns7&id=o-AOJlL4S3D5y56l-J43DPJE2xmuVyCA7xgZM0K0UgOodX&sparams=clen%2Cdur%2Cei%2Cgcr%2Cgir%2Cid%2Cip%2Cipbits%2Citag%2Clmt%2Cmime%2Cmm%2Cmn%2Cms%2Cmv%2Cpl%2Cratebypass%2Crequiressl%2Csource%2Cexpire&ei=pYhJWeS5HcutWMyzlKgF&ip=2001%3A19f0%3A7402%3A95%3A5400%3Aff%3Afe6a%3Ad50a&gcr=us&requiressl=yes&pl=47&expire=1498012933&mm=31&gir=yes","urls":""}
Ver esta pregunta y respuestas, esto podría ser atento: http://stackoverflow.com/questions/2678051/cant-download-youtube-video – COD3BOY
posible duplicado de [Descarga de vídeo en formato FLV de youtube ] (http://stackoverflow.com/questions/4287748/downloading-videos-in-flv-format-from-youtube) –