There is no “the best font” for email. It more depends on the country and language. According to the eSputnik marketing team, the best fonts for emails for Russian-speaking countries are Arial and Tahoma.
If we are talkind about English-speaking countries, the most popular font is Helvetica. Arial is on the second place.
Here I attach some others email safe fonts:
• Times New Roman
• Verdana
• Courier / Courier New
• Tahoma
• Georgia
• Palatino
• Geneva
Using this email fonts, you can be sure that they will render in users’ inboxes exactly like you planned.
You can use custom fonts in email. But, you should be using a font stack, which includes those fallback fonts in case the inbox app doesn’t support them—graceful degradation.