DLC from LL, from best to worst
Kshatriya - Polis Ranger Trainee (faction pack)
Spider Lair (developer pack)
Red Line Sniper (faction pack)
Pavel (Chronicles Pack)
Khan (Chronicles Pack)
Anna (Chronicles Pack)
Tower (Tower pack)
Reich Heavy Soldier (faction pack)
The best was Kshatriya, a big level where you play a hybrid open world Metro/Stalker, where you explore at your own pace taking loot and valuable artefacts left in the civilization above the sewers, but you have to take care of the radiation and the need of filters (and you won't find filters left around this time!), which works great with the economy system implemented that allow you to buy new stuff. As you explore you open new pathways that serve as shortcuts that connect with other parts of the sewers, and the semi randomized loot and monsters round up the experience.
The feeling of exploring some unknown dark place with mutants, radiation, searching frantically for valuable books or components while your clock ticking warning your filter will consume soon and you aren't very sure if the time left is enough to come back all the way or you should push forward hoping the next exit is at the next corner (but it isn't) is great.
I had to lower the difficulty to medium, it's pretty tough, even if I played the rest of the game in hard.
In fact, I will venture to say this mission has too much work put in it in comparison with the other missions (though it recycles the famous Library level), I suspect this was actually a prototype for the next Metro game.
Spider Lair was also great, the developers really nail the feeling of being alone in some underground, dark, abandoned, claustrophobic cave/sewer/base/metro passage, illuminating your way with a flashlight, some eww!-worthy monsters and eggs, burning webs with your trusty zippo. It's all linear, but the atmosphere is so thick and it all works so well you don't care. In this mission you are given a flamethrower and lots of spiders, and believe if you get to use it well.
The following three missions are all your bog-standard fare for Metro missions. Good, but nothing new.