It really depends on what one considers "effective". For defense, the best "bang for the buck" is a Medium Vehicle in Light Armor with Double Construction Modules set to non-combat. Infantry HCAP is a very good "all rounder", for vehicles it tends to be best to fit them with the largest weapons available and put them in numbers which would counter your formation concepts. (So if one is running two infantry divisions for ever armoured division, one would want to have a 2:1 ratio of AI v AV weapon platforms in the attacking formations, for defending it's more about weight, want as much "pressure" on cheap infantry which are fortified as possible, so usually go for a 8:1 ratio of them to anything else in their formations.)
Infantry in Infantry, Motorized, Mechanized, or Armor units do not slow them down at all, giving them Light Weapons and using them as a damage shield is a viable, if not cruel, tactic.
FFD's set to non-combat will rarely get hit as infantry, Light Vehicles is fine for attacking units though, just expensive.
Static heavy, primarily cost, especially with ARTY. Since ARTY is normally not on the Front Line duty, it means either only ARTY is hitting it or there has been a breakthrough. If the latter, no amount of armour will save it, for the former, depends on enemy composition, I find it not worth it on anything other than STO's.