What is the best Separator to Separate Multiple Emails?

The separator should be a comma (,) and there should not be a space.

See RFC 6068.

Here's a late caveat in case anybody needs it:

Even though RFC explicitly recommends a comma, Microsoft Outlook will use the "list separator character" defined in the regional settings. Your mailto links may not work correctly for your Windows + Outlook users whose systems are configured with a different list separator such as semicolons. Outlook will simply refuse to split the e-mail addresses with commas.

Just something to keep in mind.