Literally cannot be anybody signed after 1986.
Yes we've had plenty of 'bargains' since then; £11m Yakubu whose goals got us into Europe (but we didn't win anything), £1.7m Cahill who become one of the top attacking midfielders in the league for half a decade (but we didn't win anything), £30m Pickford who saved us from relegation twice (but we didn't win anything), £6m Baines who was one of the best left backs in the Premier League era (but we didn't win anything). Even £850k Graeme Stuart whose goal saved us from relegation.
But obviously it's Southall.