Looks like they had adjusted the tiers, t5 use to be up to gs256, but now it's listed as gs256+ and I have noticed enemies taking more damage to take down. I would imagine everyone is running resistance these days and not much on missions. Missions are only good for phoenix farming these days (gs and exotic farming if running lexington), veterans really only need a lot of phoenix for gun talent respecs.
For lower levels best I can think of to quickly get set items, do the high value target missions, if you can run as a d3-fnc build which is a monster of a tank build and with the right attributes in place (increase armor damage, increased elite and named damage etc) and decent smg (ferocious, destructive) you can take down elites easier. You should be able to solo challenge and hvt's with fnc build.
You can farm phoenix in places like lexington, which gives 80 on challenge mode, it can take less than 10 mins to run, again the fnc build is a monster here. You can then use the phoenix to buy the individual pieces they have for sale or the gear set caches at base, which apparently now also have a chance to drop classified.
Farm the week long global event starting tomorrow, you can buy the event caches that will give you a good amount of gear sets and classified.
If you want, you can also farm resistance for it's cache using this method, do the resistance as world tier 1, the mobs will be level 30 gs0-164(or something like that) so the gear that drops in there will be the same as the world tier you're playing, but, the cache you get from beating the boss wave every 5 levels actually drops items that match your current gs and you can share those items. You don't usually get much in terms of gear sets as drops in there anyways, as long as you beat wave 10 your cache will be tier 2 which has a chance to drop exotics and classified too.
The new zone will spawn random event missions now for you to do, even while in the the resistance and awards you with a west piers cache when you complete them.
You can farm the named bosses around the map. (I would suggest this for some quick kills and names now always have a 1% chance to drop exotics, and 3% chance to drop classified). I don't remember the respawn times for them last I heard 4 hrs, but some people say to jump servers, not really sure how to do that other than maybe join another friends game or maybe changing the world tier.
This is who I check out builds from
I still run an alpha build close to this in duo, and fnc for weaker groups
Vendor weekly listing
The Division: Weekly Vendor Reset
Be sure to check out the purple mods that have % dmg to elites, they're useful in some builds, good to have a set of each stat around.