Southampton.
In two seasons he's signed:
Cédric Soares £4,700,000
Juanmi £5,000,000
Cuco Martina £1,000,000
Jordy Clasie £8,000,000
Oriol Romeu £5,000,000
Virgil van Dijk £11,500,000
Charlie Austin £4,000,000
Dušan Tadić £10,900,000
Graziano Pellè £9,000,000
Fraser Forster £10,000,000
Shane Long £12,000,000
Sadio Mané £10,000,000
Florin Gardos £6,000,000
Ryan Bertrand £10,000,000
Net spend: -£22m
His weaker signings were mostly very cheap, but he signed Clasie, VVD, Tadic, Pellé, Forster and Mané for under £50m. Mané alone has just gone for over £30m. In a short time he's brought in a top keeper, forward, fullback, centre back, playmaker, winger and centre mid. Even the likes of Long have proven worthy signings for them. His ratio of good:bad is clearly much better than 90% of managers in the league, and the fact that they've accomplished this with a negative net spend shows some astuteness (though how much of that is on Koeman's end I'm not sure).
What I find encouraging is that he's clearly got an eye for a player in every position. Bar maybe right back, seeing as 3 of his weakest signings have all been right backs!