Mafia - Most the game really but parts that really stand out are the assassination in the brothel, which leads to the roof top chase, which leads to the funeral shoot out, which leads to the Police chase. Jesus I felt like I'd been beaten up by the end of it.
The ambush at the restaurant was classic too.
H&D2 - The bridge assault in Sabre Squadron was great as was the holding the Oasis. Hearing your first mines go off and having to leg it over to the breach to machine gun the enemy was great.
Vietcong - The radio relay defense/extraction. Flying in with a MG blazing then being dropped ontop of the hill battling off dozens of VC, in the pouring rain and having to drop your gun and swap to Shotgun or nade launcher as ammo runs dry, now *thats* a gun fight.
S.T.A.L.K.E.R - the army and monolith guys ripping each other apart as the meltdown begins. Especially with the realistic damage mod, that gun fight gets real intense real quickly.
Call of Cthulu - having no gun for ages is freaky enough, but when armed when are smashing the **** out of your hotel room door and you're forced to feel through windows and roof tops you practically have a heart attack.
OFP - playing the IRA custom mission with my flat mates on LAN was ace. One group plays US rangers assigned to wipe out an enemy convoy, another group plays the hostages in the convoy who have to survive the ambush and gather arms, and the third group play SAS charged with clearing the beach head after flying in on helos. Then you all regroup to assault a huge defended castle. Add in usable mortars and you've got one hell of a battle! Sooo tough though, we never finished it, even when we figured out how to use the mortar and had two of us relaying targets
Ghost Recon: Advanced Warfighter - finally reaching the embassy just to have it explode in your face. Then taking to the rubble to hold out the enemy assault, including those ****ing pain in the arse tanks! having to laser target them just to have a shell hit me in the face drove me insane! One hell of a gun fight though, especially when you have to carry VIP2 to the chopper while under fire and then get told theres no room for you and you have to wait for another