for weather, probably north of San Diego. They average something like 10 rain days a year. And in the dead of winter you can go out in knickers or shorts, you just have to wait till 11 AM. I've already booked a flight for Feb 07.
Other areas which fit your criteria: Bear Mountain NY, upper CT (Barkhamsted Resevoir), virtually anywhere in VT/NH.
however, when you ask about the best place for road cycling in the US, you seem to imply that smooth roads, no traffic, and climbs are desireable.
I find city rides the most fun. Therefore I submit two of my favorite places to ride:
1. NYC, midtown Manhattan (Greenwich village up to Central Park) - esp with synchronized lights and some semblence of awareness of cyclists (due to the messengers). The fact that traffic rolls at 30-40 mph between lights means you constantly have ideal motorpacing vehicles around. And when they get caught in slower moving traffic, you can jump around them to catch the vehicles "breaking out" of the temporary jam. The best is doing the run down Broadway, with its more interesting diagonal intersections. There is never a dull moment when riding in the evening, say from 5 to 8 pm.
2. An unidentified city in SW Connecticut. There is a 2 mile loop through downtown which has one way or streets with medians. So you can ride with traffic and not worry about any oncoming cars. With the available street lights, the loop (and its ancilliary loops nearby) is ideal for late night sprints, using cars and trucks as leadouts. Even the occasional police car assists either by following or by leading out. The drunk bar patrons only help by giving some atmosphere, cheering, telling you how fast you're going, and yelling from the outdoor patio areas.
cdr