Coutinho MAYBE? £8m was a steal of great magnitude. As Dee points out Hyypia and McAllister were amazing signings.
Best one ever I hear you ask? Kevin Keegan.
Shanks signed him for £20,000 in 1971 (for perspective; at that stage the British record transfer fee I think was £180,000). When we signed him there were 5 teams battling it out for supremacy in the top division Derby, Arsenal, Leeds, Liverpool and Man City (1 point separated the top 4 at the end of the 71/72 season). If you look at the domestic trophy winners from the late 60's to early 70's there was no one dominant team.
Keegan was a talisman, he was the missing link, the focal point and the heartbeat of the team. He (and Shanks who signed him) IMO was the catalyst for our 20 year domination of English football. We had other great players at that time of course, Hughes, Callaghan, Smith, Toshack, Clemence etc but Keegan (twice winner of the Balon D'or*) had that extra bit of magic.
He was there (and scoring) in our first ever European success (UEFA Cup winners 1973) and was inspirational in our first ever Champions League* win in 1977.
His big misfortune is that when he left us in '77 his replacement, Kenny Dalglish, went on the become our greatest ever player. I think it's for this reason that Keegan regularly get overlooked in various "greatest LFC teams ever" or "greatest ever LFC players" debates. Personally I think he's the 2nd best player to play for us, putting him ahead of the likes of Hansen and Souness etc..............that's how good he was.
* Balon D'Or and Champions League weren't known by those names back then.