I've always liked
Isobel. I prefer it to
Isabel because that's closer to Isabelle/Isabella.
Isabelle and
Isabella I know are popular, despite me never having heard of a little girl in my area with either name, but it doesn't really ruin
Isobel for me. But that might be because even though I think it's a pretty name, I don't think I'd ever use it in real life (there are a few more I'd use first).
I think it'd go best with other pretty, more classic names:
Isobel CatherineIsobel ClaireIsobel EdithIsobel EvelynIsobel FrancesIsobel GeorgiaIsobel MargaretIsobel Vivian