If my work is going to only be used to create image files that will be used online, or in email messages, what is the best Pantone swatch for me to use?
Pantone is only actually used to reliably recreate colours in offset printing. If the end use of your product is RGB, then RGB colours are all you need—just like AAGD suggested in the comments. Pantone swatches are not something you should use or worry about in on-screen-only designs.
Of course, you could still use a Pantone colour book to find a colour you like and then use a conversion table to use a similar hue in your design. As to what Pantone hue you'd use, that is entirely up to you and depends on your design, style and tastes.