I test rode around 15 bikes this summer, and the Trek Emonda (in 500 level carbon, so the "SL" models, they start around $2,600) is better than all of those.
The Roubaix was surprisingly stiff for an "endurance" bike. The Synapse was fine, better than many bikes, but nothing amazing.
The Emonda was rather amazing. I specifically ran over bigger and bigger potholes, and the bike just floated over them. It both remained stable, and your hands weren't jarred. You should check it out. Like I said, ride the 500 level carbon models (the "SL" frames, not the "S" frames).
Another possiblity is the new Specialized Diverge - it's not out yet, don't know how it's ride compares to the Emonda. It does take bigger tires (more cyclocross level clearance) though.