My vote is for Douglas
I crossed at Douglas back in June and it couldn´t have been any easier. I think I crossed mid-day on a Thursday and there were no lines. I was probably back on the road within a half-hour. You do everything in one office and the people will direct you to where you need to go. You´ll have to make copies of license, registration, passport, etc, but you make them right there in that same office. I haven´t done any of the other border crossings into Mexico, but it really couldn´t get any better. On top of that, Agua Prieta (sp) is small and you´re out of the town in 5 minutes. If you cross mid-day, there´s a hotel in Montezuma, a couple hours south of the border, for $20 with secure parking. Hope this helps.