If you use Free Market economics, then the best units in SMAC are derived from customizing Probe Teams in the Unit Workshop, as a token defensive unit in each city and for sending out to hold on key squares your opponents might choose to attack - but never in a unit stack, due to all the loses that could happen.
*they cost no support to maintain in the long-run - saving industry resources for your cities to produce other units.
*they double up as probe defense because they are considered probe teams afterall.
*their mobility can be minimized in the unit workshop saving on their cheaper cost to produce them. very economical.
*their inability to police a city doesn't matter because there's no bonus to police from using Free Market.
*their defensive capability can be upgraded from key technologies, so that they are effective at defending (except in a stack of units where it can be an all-or-nothing game).
*they can even be placed in your opponent's territory, egging them on, to attack them, and don't count as a military unit, so its ok to send them there even with a Free Market, not contributing to a city's unhappiness.
*they can be over-produced and even "sacrificed" at your opponent's cities for stuff like technology, or even just Hold their position on key resources their city has, so that their city is sieged.
If you don't use Free Market economics, then, I'd recommend Scout Patrols with Non-Lethal methods from Intelluctual Integrity tech (doubling their policing capability in cities) as a best unit for the policing they do effectively. Put 1-3 of these units in each city, depending on your Police rating, and that's the best way to keep order in cities, economically for the cost of the unit.