It's probably pretty obvious I'd go with
Violet. I think it's a beautiful floral, not too frilly or silly. It gives me a vaguely Victorian vibe that I like. I also like the possibility of
Letty as a nickname, and don't mind
Vi or V either.
My second choice would be
Ivy. It has a sophistication to it that's nice. But I am slightly bothered by the "poison ivy" association.
Daisy is one of the few florals I don't much care for. I think it's a bit juvenile.