It's hard to answer that,I have had quite a lot of different species but the success rate depends more on what is wrong when they get here than the type of bird they are. For example, hatchlings and nestlings aren't too bad whereas fledgelings with cat bites I tend to lose a lot of because of infection.
My favourites I suppose are baby pigeons and of course, corvids, they are just darling. I think it is the enthusiasm once they get used to being fed that gets me.
Having said that, they are all darlings and they do seem to know that I am trying to help them.
Each year is different with different species being predominant...that's one of the things I love about doing the rescue work, you never know what is going to arrive next.