Of the dragons that one must fish for, I think I've only gotten a Dratini in the Safari Zone, and since raised it to evolution.
I seldom venture into the Safari Zone, probably since I'm more interested in capturing all the Unowns in the Tanoby Chambers (once found a shiny X one there, but unfortunately didn't have any balls with me); but like a lot of people my bet's on Chansey, since it often runs no matter what (I've at least been in the Safari Zone enough times to know that).