I use Dropbox and VLC to watch movies with subtitles on my Iphone. (Imovie is incredibly primitive.) I use Avidemux or FFMPEG to compress them and save space.
Definitely need subtitles, too many mumbling actors and actresses these days. Amazon was genius to put subtitles on its movies, At least the ones I have seen. I suppose Netflix does the same but have never used it for streaming movies. Aside from Amazon streaming....I also use VLC and add subtitles (easy to do w VLC) snagged from subtitle sites.