A very hard call, but the heart (and the mind) says Alan. In his prime he was really the best striker I have seen, maybe bar Ronaldo, although they are of course completely different types (would be the ultimate fantasy to watch these together in their prime). I am very much a fan of Didier btw.
Have to say, this thread and a lot of others on this forum seems to be very based around the opions of the poster 'Ronaldo'. Guess you already know that, but it's coming to a point where I'm doubting if I'm going to newcastle-online.org or Ronaldos-opion-about-newcastle-online.org.