The shooter with the most difficult and interesting firefights

1 Last of US (especially the multiplayer pretty hard to play, IMHO the most realistic shootout)
Uncharted 2 Series
The 3 Series Gears of War
4 Max Payne 3
5 Tomb raider 2013

The first F. E. A. R. Oh and some budget Russian buffoon on the hardest difficulty, they're all complicated :-)

Call of Duty series
Series F. E. A. R.
Series Far Cry
Series Half-Life
The Series Lost Planet
Mass Effect Series
Max Payne Series
The Resident Evil Series
Series S. T. A. L. K. E. R.
Series Serious Sam
Series Sniper Elite
Series Tom Clancy's Splinter Cell
Series Tomb Raider

PS. My personal opinion

CoD MW 1, 2, 3 passed on the veteran. Regen HP save. Passed without that problem. Was problems little income in some places. For me now more Metro 2033 Redux brings more problems. Bullets leave rules. Need purely in the head with 1 shot. Then will be the norm. On the light in the Subway is no such problem. I myself love complexity. Now games are all simple, but sometimes appear complicated.
As said above, it is best to pay for the Russian shooter. I scolded our games. Graphics miserable, optimization, but now it is hardcore unlike games. now.

Yes, tibya Batiskaf
The first FEAR I remember very good AI bots, at that time he was probably the best in the history of shooters.

777Анатолий777
I think you just listed all the top shooters-franchise Posledny times. For example in the same Resident Evil not so much gunfights, corny, for the reason that enemies have almost no firearms, though the battles are very tense, not deny it. And for example the mass effect series and cod IMHO has never been a particular tension, even at high difficulties.

Difficult shooting? And how important is realism to you?
Difficult only the first Far Cry and this is the first that comes to mind: http://www.playground.ru/games/far_cry/ . Here where-where, and then really hard and interesting.
If more realistic, and tactically difficult (need to think a lot before doing something), then: Arma 2, Arma 3, Tom Clancy's Ghost Recon Advanced Warfighter 2 (GRAW2) and this could attribute to the mod Project Reality for Battlefield 2, but it's only multiplayer, story campaigns there.

From FPS - Far Cry, Battlefield 3,4, Arma 2, Halo.
From TPS - Gears Of War 2, Max Payne 3, Mafia 1.

Alien rage

Difficult not always interesting. And interesting too not necessarily difficult...
For me personally, the most interesting battles in Unreal, Half-Life and Crysis and Halo: Reach. There is every single place you can go in a lot of ways, that's what I value in gameplay the games the most. If you can insert yourself make life difficult by some beautiful trick-or invent an additional condition. In many places you can slide fast if you don't want the extra time to get in touch with... Well-designed freedom can decorate any game.
Of course there are more shooters such as Stalker, but skirmishes with the enemy there is rarely, therefore, do not bring them here.

F. E. A. R. first and Max Pany all. And not even discussed.