I always based it on a combination of some of the later 4E Missions and the sample grunts in War!(bearing in mind I still focus on 4E, haven't delved much into 5E yet).
Police SWAT units, like Lone Star and Knight Errant, would probably use SWAT armour, since it's designed for mobility and versatility. Light infantry in a military, like scouts, gate guards, etc, would probably use armored vests or jackets, but still possibly with military style mods, maybe a camouflage reuthenium pattern or the like. Again, for scouts and such mobility and stealth is key.
For regular infantry, we're talking about the average soldiers that make up the majority of a military, I felt light military armour was it. It's suited to all types of combat without being too specialized. For spec ops units, depending on their role, possibly medium military armour, or rather even a full body suit and PPP system, all with chameleon coming and thermal dampening and such. Again, this depends on the role, whether they're assault or espionage or what have you.
Heavy military armour is basically power armour, since it can have augmented strength in the suit itself not limited by metahuman maximums. The only time I think anyone would see heavy military armour would be only for the duration of a specific operation, and it would be heavily customized for that task. It's far too cumbersome and expensive to justify anywhere else. So heavy fire support for an assault on an enemy stronghold might have an assault cannon, grenade launcher, and foot anchors or something.
In the Operation Anchorage DLC for Fallout 3, there's a part where they get the new power armour and they form special units that operate as living tanks and artillery for a final assault on a really heavily fortified enemy base. They guys inside have a specific role to fill, and that is breaching the defenses so the regular units can get inside to do their own jobs. But it's pretty fraggin' rare, and I can see heavy milspec armour being used in maybe .5 or 1 % of all operations in a campaign.
