IMO it is almost impossible to choose who has been the best bowler. There are no obvious stand-outs in bowling as there are in batting.
You could name 2-3 great bowlers from every country and still have arguments.
I grew up watching Lillee bowl, and even modelled my own action on his, but I wouldn't say he was the greatest bowler who ever lived (Possibly, one of the top 5 ). I thought Roberts and Holding were wonderful bowlers. Ambrose was incredible on his day, as was Hadlee, Imran Khan, Wasim Akram, etc.
McGrath will be the equal of Lillee for me, in some ways I thing he is a superior bowler.
As far as spinners go, I would find it hard to separate Warne and Murali because they are both so different. In saying that, I have read a lot about O'Reilly, but having not seen him I can't judge him.
If I was in a corner and had to give my vote...I'd give it to Warne...just!