I would like to know what is the best online teacher certification and what is the site that offers it.
Very hard to answer without knowing the criteria, and even then I know of no exhaustive objective comparative study that would answer the question. It depends largely on what you would like to achieve with the certificate. Are you planning to teach for a short term or longer?
The online training market is pretty much unregulated, with lots of dubious self-accreditation and courses where the cheque not bouncing is all that is required. The in-house training courses could be useful in the short-term but if you are looking for a tranferable qualification, then these may well fall short. I'd steer clear of weekend and taster courses, too; some sites claim that you're getting a proper teaching qualification, many schools will not agree.
One of the constant criticisms of online courses is that they don't have a Teaching Practice (TP) component. Some courses are on offer that have a video TP element like this: The ICAL TESL/TEFL Certificate with TP
If a course does this, I would regard it as a strong plus because without it, online courses will always be the poor relative of traditional courses IMO.