From the history of our club Shanks and Paisley have to rank highly obviously. I think Houllier was a good manager though, I hope nobody forgets his legacy for us.
He laid the foundations for our Champions League team, and brought many good players to the club, and brought us 2nd place with 80 points, still our Premiership high point.
As for Benitez I have been impressed with his buys, Sissoko (a snatch from under Everton's noses), and Alonso - arguably his finest buy. But also Reina who has brought stability back to our number 1 shirt for the first time in a decade I would say. Agger and Kromkamp look good buys and Garcia, although maybe lightweight, has been a key goalscorer for us especially in Europe. Gonzalez looks class, and Kewell is a different player.
We are only a trio of players away from having a genuine title contending side.