Very hard. Gavaskar is probably the best one even if i don't like his style
Among those i have seen play full test matches:
Smith- 9/10 (weakness to left arm bowlers keeps him from a perfect score but great player and immensely tough)
Hayden- 8.5/10 (great vs spin, decent vs pace, relatively weak record in Eng, NZ, SA keeps him from getting higher)
Sehwag- 8/10 (dominant in sub cont but weak in 3/4 main non sub continent nations)
Cook- 8.5/10 (struggles vs quality pace at times)
Kirsten 8.5/10 (played in a tougher era, gritty, determined, not an exceptional strokemaker though. However, his record in a tougher era compared to the others puts him up there).