If I were living in TJ full-time, I'd probably choose Agua Caliente for day-to-day livability, but there's others who have a much better idea of such.
Zona Rio seems fine, especially since the police appear to try and keep all the Zona Norte junkies West of the San Ysidro Border crossing (for the most part).
*Mostly sticks near Border, talking-out-of-ass a decent part of the range*