Difficult. I would put KP first and Cook last. KP had outrageous levels of skill.
Boycott was technically the best against all-comers, but a slowcoach and England needed shot-players to follow him.
Gooch was excellent for his last three years but fairly average before, other than against WI. He had some difficulty against medium pace swingers.
It's difficult to place Stewart as he only played fifty tests as a batter, but he was excellent against West Indies and Pakistan in that role. Brilliant timing. He had trouble against good spinners such as Warne, but to be fair so did a lot if people.
Gower was arguably the most complete of all with timing that had to be seen to be believed. But one got the impression that he could have averaged 50, with a bit more focus, than the 44 he ended on. Excellent against spin.
Thorpe was quietly impressive and averaged 50 in the reactive position at #5. He was key to England's come-from-behind win in SL, nullifying Murali.
Cook has piled the runs up in an era of less strong bowling but struggles against top quality quicks. Much better against spin.
My top six would be
Boycott
Gooch
Stewart
Gower
KP
Thorpe