Agree with the general consensus on some but:
Shankly - Yeats & St John. These were the 2 most important signings in the clubs' history. They kickstarted everything Shankly did which dragged us into our glory era.
Paisley - Rush. Kenny was the better player but signing a guy from Chester City who went on to score 346 goals...
Dlaglish - McMahon. We'd gone through a mediocre 84/85 without replacing Souness. Kenny did it with his first signing. McMahon was an awesome player and the heartbeat of that wonderful 87/88 team.
Evans - Scales. This signing signified the change in formation which defined the Evans era and brought 2 and a half years of truly memorable football. Scales was the best player in 94/95, in my opinion. Also, it's becoming a common myth that that team couldn't defend but look at the stats...one of the meanest defences each year with Scales there.