Helena has too many pronunciation possibilities. I much prefer
Helen or
Elinor.
Rebecca is probably my favourite, though I don't enjoy
Becky much.
Becca is OK.
Tabitha is way too feline - unlikely to be used in full, it can only shorten to
Tabby! I love
Talitha and its impeccable feminist history, and I've actually met a couple, though I've never met a
Tabitha that didn't say meeow.
Olivia is a puzzle. I love the sound and the Shakespeare connection, but Olly ... yuck! And a distant relative of mine was named
Olivia in about 1910 I think, and insisted on being called
Olive instead. Which I don't like at all.