If I had to take a stab at busiest freeway exits to surface streets, I would say:
In the L.A. area, I-105 WB exit to Sepulveda Blvd north, which takes the bulk of LAX traffic and is very frequently overburdened.
In the D.C. area, I-495 at VA-7 which is in Tysons Corner. I-395 near the Pentagon also has very heavy exits, but many of the Pentagon ring roads qualify as expressways.
In NYC, I-278's exit to the Brooklyn Bridge is also heavy since it provides a direct connection to Manhattan and avoids the toll on the Battery Tunnel. I don't know if the bridge is "expressway like" in the context of this thread.