I'd say it depends on who you're talking about. For example, it makes no real sense to increase RG's attack, but it does to boost his Health/Protection or Defense. Rey, on the other hand, greatly benefits from Health/Protection boosts to get her out of the glass cannon category.
HK-47 is probably in the middle. Increasing his damage could be useful, but he's pretty much a non-taunting tank for Droid teams. Honestly, I'd probably pick Health/Protection and then try to get secondary stats that boost Offense. Another route would be Potency. His AOE would greatly benefit from that.
Honestly, Health Mods are the way to go right now. Unless you have all the First Order characters unlocked and built up, you're not getting MK5 Offense Mods any time soon. And you should shoot for at least MK5. Raids have a chance at 6 and 7, but those are incredibly expensive to do and it's all random. Unless you get lucky, it'll take a while to build up anything other than Health (Any), Defense (Jedi), or Tenacity (Rebel) sets for a while. Jawa teams are a long way off. Scoundrels could work, but there aren't many of them that were worth truly investing in before. Other than Rey, Resistance has been pretty useless for a while. Empire is pretty useless outside of Royal Guard. So until you get any of these teams together or just get lucky from raids, Health Mods are the way to go for now.