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Mathew Hayden was opener, Andy flower was WK Batsmen. So their comparsion with Smith/Williamson doesn't makes sense anyway as both Smith/Williamson are middle order batsmen (1st -2nd down), Hashim Amla is their competitor (Jacques Kallis and Viv Richards are irreplaceable) but if i have to choose all time XI then will pick Smith over Amla anyday, his average against India is outstanding.
I have not seen Viv Richards play but seen some highlights and a few documentaries about the old West Indies team and they had some incredible players who will get into any team and Garfield Sobers who is regarded as the best allrounder of all time and everyone including Jacques Kallis is placed on his level to compare and the fast bowlers were deadly.