I’m sure a lot of people here will comment on his Cold War novels and their TV adaptation, but what is truly impressive to me is how he managed to switch gears mid-career and write blisteringly powerful indictments of the modern world. The little drummer girl (there’s a good BBC series of this, if you liked TTSS or NM check it out), Absolute Friends, the Constant Gardener..
I remember a review for the Little Drummer Girl that said (paraphrasing): "This book has achieved the rare miracle of making you feel sympathetic to both the Israelis and the Palestinians."