Elisabeth. Normally I go with the original / more common spellings, but
Elisabeth looks softer and prettier to me.
I like
Ellie and
Bessie; I might use them both. I have a friend named
Elizabeth who goes by
Beth, so that one is out.
Liz looks too grown-up and dated to me.
Bess is okay, but I prefer
Bessie. I had a friend who just went by
Elizabeth and never used a nickname - I like that option too, but I compulsively nick-name cute things like babies and rabbits, so I'm sure I'd have to use one.