Donald, hum. What goes against him for me, in comparison to McGrath in particular, is that he bowled pace in 1990s South Africa, possibly the best possible time and place in history to bowl fast. I'm trying to pick out how much of what you say was Donald's unique ability- there's no doubt that he had it in abundance- and how much was the conditions he played in. To take an example, you wouldn't say that Makhaya Ntini is a swing bowler. In fact, you've previously said he isn't, and he uses swing no more or less than McGrath did. But only last week, he bowled a startling inswinger to Ponting that pegged his off-stump when he was offering no stroke.
I don't doubt Donald's ability, but he had something to work with more often than most. What amazes me about McGrath is that he bowled at one of the worst possible times and places to bowl pace- Australia, '93-'07. Ever since the turn of the millennium- or September 2001, if you prefer- there's been not one fast bowler who has gotten on top of batsmen consistently for any length of time, bar McGrath. From September 2001 until his retirement, he took 197 wickets (non-minnow) at an average of 22.
Has a fast bowler ever risen so highly above his contemporaries? Is there any other bowler in history who has had to face more flat pitches and more batsmen averaging over 50 than McGrath, and come out of it with a record like that? There sure aren't many.
And you can say you don't doubt that Donald or Ambrose would have surely done just as well, but you're merely speculating. Just as when I say I think Hayden would still have averaged over 40 in the nineties, I'll find people like you who disagree. McGrath is the bowler who dominated when no other fast bowler in the world could, and that's an indisputable fact. So when i'm picking a side, any side, i find it impossible to look past him.