what is the best sealant for leaky pipe?

You omitted to say what the pipe contained. If it is mains water for example, it is better to have a section inserted, or totally replaced. If it is discharge water from a shower or sink for example you can get away with an epoxy patch and some bandage , large DIY stores sell a 2 part type.

If your speaking of the pipe it self I don't think any of the sealers are good on a pressure line. A lot of the epoxy type sealers work on drains but you have to clean and keep dry.To conclude if your speaking of a thread sealant teflon tape is my choice for liquids.

Depends on the pipes use. For a waterline, you should replace the bad section. Patching can fail. On drainlines, you can use epoxy or a rubber patch with a radiator type clamp.