W       X       Y       week    Z
w1      x1      y1      1       0
w1      x1      y1      2      0.1
w1      x1      y1      3      0.2
w2      x2      y1      1      0.5
w2      x2      y1      2      0.7
w2      x2      y1      3      0.3
w3      x1      y1      1      0.1
w3      x1      y1      2      0.2
w3      x1      y1      3      0.5
w4      x2      y2      1      0.7
w4      x2      y2      2      0.3
w4      x2      y2      3      0.1
w5      x1      y2      1      0.3
w5      x1      y2      2      0.1
w5      x1      y2      3      0.2

cast(foo, W + X + Y ~ ., max, value="Z")

W       X       Y       week    Z
w1      x1      y1      3      0.2
w2      x2      y1      2      0.7
w3      x1      y1      3      0.5
w4      x2      y2      1      0.7
w5      x1      y2      1      0.3

library("plyr")
ddply(foo, .(W, X, Y), summarise, week=week[which.max(Z)], Z=max(Z))