600EX RT is the best speedlite for 5D3. I have two of them, 430EX II and used to have 580EX II before. 600 is easy to use and it just works. Flash head must turn 180 degrees both sides, which 430 can't do -> 580/600 if you will buy canon.
600RT or even older 580 EX II will be very useful. At $373 for refurb 600RT, grab it.