My problem with THE DA VINCI CODE was that, although it was a page-turner and you have to give him credit for the cleverness of his plot, his characters struck me as cardboard cutouts. The guy, in particular--can't even remember his name--is so passive that it was hard to maintain any real connection with him. And if there was supposed to be sexual chemistry between them, I sure didn't feel it. Not sure if I'll give ANGELS AN DEMONS a shot, but I might, since the movie's coming out.
If you like historicals with urban US settings, try Barbara Hambly's Benjamin January series, beginning with A FREE MAN OF COLOR. For my money, it's the best historical mystery series ever written. Nice, complex mysteries featuring a fascinating sleuth set in 1830's New Orleans.
And I'm outing myself here, but you might also check out the Gilded Age series I wrote as P.B. Ryan, about an Irish-born governess in post-Civil War Boston. The first book is STILL LIFE WITH MURDER, but I'll warn you, the early books in the series are only available used, and that one goes for a pretty penny. Someone's actually trying to get $125 for it, but he's delusional.
Another great historical mystery is ROSE by Martin Cruz Smith, although the resolution might leave you going, What the &^%$? You might also check out Daniel Stashower's THE BEAUTIFUL CIGAR GIRL, which is the true story of the first big, splashy murder in NYC, that of Mary Rogers in the 1840's, which Edgar Allen Poe set out to solve. Really. Very cool story. It's also a fascinating portrait of Poe.
As for contemporary suspense, you mentioned Koontz, but I found THE GOOD GUY to be a bit of a disappointment. Recent reads that have really spun my wheels are Chelsea Cain's HEARTSICK and SWEETHEART, Harlan Coben's PROMISE ME and THE WOODS, Tana French's IN THE WOODS (which has a very similar premise to the Coben book but a different treatment), and Dennis Lehane's SHUTTER ISLAND--which Scorsese is making into a film called "Ashecliffe" starring DiCaprio, to be released Oct. 2. I can't wait.
