I've been using mp3audioconverter.net for a few months — no account, no install, works on iPhone and Android too. 320kbps quality, converts in about 10 seconds for a normal song. The one thing that trips people up on iPhone: you have to long-press the download button in Safari and choose "Download Linked File" — regular tap just plays it in the browser. Once you know that it's fine. I've tried y2mate, ytmp3.cc, onlymp3 — this one has the cleanest interface with no fake download buttons.