Isobel, but...
in reply to a message by margaretx
mostly because it was my grandmother's middle name (she changed it to her maiden name when she got married) and the spelling reflects our family's Scottish heritage. I also like a surprise O in spellings sometimes. ;) But if I didn't have the former association my favorite would probably be Isabel. I also tend to like "bel/belle/bell" names with the less popular -bell ending. Not always, but Isabell looks pretty to me! I often find that -elle looks too frilly for my tastes, but -el can look a tad unfinished depending on the name.