I will do one for seam bowlers. I will let Richard do the one for spinners as I will cover what I think makes a good Test bowler. This has come across to try and work out Marc's reasoning that Flintoff was England's best bowler against South Africa.
Here is what I believe makes a good Test bowler:
Swing the ball both ways or be able to bowl a good outswinger or a good inswinger, a good seam position, patience, accuracy, pitches the ball up rather then banging it in short (you have better chances of getting wickets of proper balls), able to get cut both ways and if possible - able to bowl reverse swing.
Now Marc, does your beloved Flintoff have all of these qualities?