Among the people involved in pop music industry including the popstars - both vet and new ones,I think Britney.A lot of them acknowledge her influence irrespective of other things.Among the public,umm..I don't think any of the people will reply with 'Yes,I do' if they're asked whether they have 'respect' for any of these 3 popstars.
But well,I know that it's DEFINITELY not Britney.People don't even think she posseses the intelligence of an average human being,let alone thinking of her as some capable musician.So it's either Rihanna or Katy.I've voted Rihanna cause straight men aren't embarassed to say they like her or have her music,atleast collaborations,in their iPods(I know that it doesn't exactly signify respect,but well.)Can't say the same about Katy,even though she's more of an 'artist'.