Well the best would be one backed by a known university right? So CELTA or Trinity TESOL. But those aren't online.
None of the strictly online ones are accredited. This is a fact simply because there is no such board to accredit them. As such, its doubtful anyone here has done more than one course, because well, an online course is just there to raise your pay level for most people; comparison thus becomes impossible. Actually, I lie, there was this one guy online who was undertaking them all and rating them for fun, but sadly I've lost the link to his blog. He was brutal though.
As you're specifically after an online course, I'd be asking people to rate the ones they did in terms of specific matters of convenience then. So whether a specific one had a convenient schedule, whether they were prompt with returning assessments, whether the course notes would pass an MS Word spellcheck, whether the feedback was useful, how professional they were, and what portion they felt was actually applicable once they got in the classroom.
Personally, the online garbage I did ticked off only one of those categories (feedback, but their assessors were all casual hires so the staff may have rotated by now), so adding it would be futile.