Loved the hell out of that level in God of War 3, and just love everything about that game overall. But my personal pick from last gen would be a mission in Halo 3 called simply "The Covenant".
First off, it starts in the clouds with a group of Pelicans flying in formation before dropping you off on a forest beach, spartan laser in hand, to tackle a few squads of Covenant infantry and an AA gun emplacement.
That's already a cool and classic Halo setup, but its just the very beginning.
Once you clear the AA guns you get an airdrop of Warthogs you can use to fight your way up a mountain pass, tackling defensive turrets, light infantry, Ghosts and finally a pair of Wraith tanks and some Brute choppers when the path finally levels out.
This leads you to the first of the two control towers to assault. Inside the first forerunner structure you fight your way through Brutes, Grunts and Jackals until you reach the elevator to the control switch. That switch is guarded by a Brute Chieftain and a full Brute pack and makes for a harrowing mini-boss battle.
You're not done yet though. Once you've defeated the Brutes and flip the switch, you head back outside and work your way to the beach - and there find waiting Hornets (single pilot aircraft that two players can hand on the outside of) and take your fight to the skies. The level opens up into a sprawling battle between ground forces of infantry, turrets , AA guns, tanks, fuel rod equipped enemies, Banshees and Covenant dropships, all spread out and guarding the second forerunner tower.
After this battle
you're still not fucking done. You've still got to clear out a landing spot, tackle the outer tower defenses and make your way into the facility proper. This time those halls are guarded by Grunts, Brutes and worst of all, Hunters. And even then once your reach the elevator you'll have a second Brute Chieftain and Brute Pack to deal with before you can flip the second switch.
And no, you're still not done.
At this point you make your way back outside and find Warthogs and a UNSC Scorpion tank. You'll need it as you push your way forward through a high and winding mountain road, littered with enemy vehicles, sniper perches and makeshift defenses. At the end of this long road of destruction are more Hornets and weapons parked at the ready. And that's when things get really interesting.
TWO huge Scarabs drop from the sky and dominate the valley below. Your job is to take both of these behemoths down by any means at your disposal. They're packing enough weaponry as is, but they're fully manned and crewed with Brutes and Jackals and Grunts and leaders, and helped out by skirmishing ghosts in the snow below.
Even after you watch the second Scarab go up in an explosive fireball, you're still not fucking done yet. You make your way into the main Forerunner structure which consists of a long series of hallways ending with a control room - the room where the Prophet of Truth is preparing to fire off the Halo rings. This long hallway is guarded by fuel rod Grunts and everything the Covenant can throw at you. Once you fight your way through, you have a final encounter with Truth and only then can you finally take a breath and relax a bit -- BUT NO -
This is when the fucking Flood attacks from every angle, reanimating those fresh enemy corpses into combat forms.
The hallway is now littered with dozens of flood combat forms lunging and firing and stabbing and moaning and exploding. Only when you fight your way through the hallways again and defeat that entire horde do you at last reach the snowy valley again and complete the mission.
The level has always astonished me in terms of scale and scope and ambition,
but its also crazy replayable and a very different experience each time. Play it with friends, play it with skulls, play it solo on legendary difficulty, play it on foot, play it with hogs filled to the brim with NPCs armed with heavy weapons, play it while trying to keep that brute chopper as long as you can. Whatever. Its epic, its insane, its sweeping and gargantuan and at times hard as nails, but it also shows off everything that makes Halo great.
It might also be one of the best shooter levels ever made. Its my personal favorite anyway.