First post. Momentous.
I'd probably give it to Brian Eno. He's one of the few producers (Albini's another) that can manage transform any band into a conduit for his own distinctive sound--notably with Talking Heads.
I am, however, prejudiced in his favor because his work outside production is so brilliant. He didn't produce Bowie's Berlin trilogy, but Low wouldn't have been the masterpiece it was without Eno.
Hell, My Life in the Bush of Ghosts alone warrants his status as one of the greatest studio experimentalists ever.
Plus, he did the music for Spore. Badass.
but if the band's music isn't good in the first place the producer can't make it good.
Perhaps. But what about Loveless? Only about half of its songs (Only Shallow, When You Sleep, Soon, etc.) would be great songs without Kevin Shield's blizzard of guitar. The rest (especially Touched, To Here Knows When, and What You Want) would be too repetitive to survive without Shield's production work.
But as I said, that certainly isn't the case for all of MBV's songs, so you do have a point.