Steyn is just in the top ten for me, based on his awesome record. I would consider him the greatest strike bowler in history, in terms of having action spells where he can quickly win a match. It is incredible that in the last seven years, he has produced a match-winning spell in almost every series he has played, in every country, with 1-2 exceptions. The fact that you can have a series begin with the knowledge that he will almost surely win a match by his lonesome is a special feat.
However, the reason why I feel he is below Marshall, Hadlee and McGrath is that Steyn often goes missing for much of a series for no explicable reason, and then produces a match-winning spell out of nowhere. This happened in Australia in 2012-2013 and against India at home last year. He suddenly lacks penetration and consistency, whereas the other greats didnt seem to have this problem.
If you look at his career, you can see this trend, producing 1-test standout performance and then just decent/merely good in the rest. The best example I can think of was in India in 2011 when he was devastating in Nagpur and then in the next text seemed quite ordinary.