unproven leg spinner in international cricket, Danish kaneria takes wickets for fun in county but has struggled internationally, so cant judge Tahir yet.
Kallis looks well past his best now.
Morkel has potential but then again so did Harmy, he could be just good enough or eventually world class, again time will tell.
I would suggest a leg spinner with 261 wickets at 34 in test cricket has not struggled.
I would suggest Morkel with 119 wickets in 32 games at a healthy average of 29.46 (signifcantly better than Broad and Anderson) has more than 'potential'