Dodgealum - I can't say I typically "buy" for a particular producer but I definitely notice, as you suggest, what seem to me consistent, successful pairings of artist & producer. Would have to dig out the recordings to give you specific names for some current candidates but a few pairings I like are those with Andy Summers, John Mayer and Levon Helm. I don't think it's possible to give too much credit to George Martin for his work with the Beatles. Tchad Blake, Phil Spector and Daniel Lanois are three that quickly come to mind for influencing the sound of an artist - maybe too much in some opinions. Interesting topic.
Influential Producers
I've always found it interesting how much a producer can influence the sound of an artist. The right synergy between the two can make a good artist great and really shape the creative process. A few influential producers come to mind: John Levanthal (Rosanne Cash, Shawn Colvin, Marc Cohn), Ethan Johns (Ryan Adams), Tommy Lipuma (Diana Krall). Sometimes I shop recordings by looking for producers who have worked with some of my favorite artists--it doesn't always guarantee a great recording but it helps narrow things down as the producers mark is usually evident no matter which artist is involved. Anyone else look at the credits and see patterns?