Gradius V (PS2) | Asteroids!
There's no shortage of shooter stages in this list, that's for sure. Makes sense, seeing as they usually only have five or so levels at all, so they've gotta make 'em count. Gradius V's asteroid-packed level is already hard, forcing you to weave your way through some ridiculously dense fields. Then, to top it all off, the boss makes you literally carve a path through hundreds of incoming obstacles while simultaneously avoiding all the crap it's shooting at you. Intense, insane, incredible.

GTA: San Andreas (PS2, Xbox, PC) | Breaking the Bank at Caligula's
Grand Theft Auto missions are famous, not only for their sheer, overwhelming quantity, but also for their incredible diversity. Games that are supposed to be about stealing cars have somehow expanded to include anything from pimping prostitutes to hijacking military fighter jets in midair.

San Andreas has the best of these wild and wonderful missions... and this is our favorite. The final mission of Las Venturas - a casino heist that CJ spends nearly 1/3 of the game planning - has everything. Gun fights. High speed chases. Explosions. Disguises. Night vision. Sabotage. Betrayal. Forklifts. And at the end, you get to base jump off the top of a fifty-story building, parachuting over the neon-lit city as SWAT helicopters try to keep a spotlighted bead on you.

This stuff is worthy of a summer blockbuster.

Half-Life (PC, PS2) | Unforseen Consequences
Okay, so you accidentally (maybe) caused an explosion that opened a dimensional rift and let aliens invade. Things can only get better, right? You regain consciousness in your special, armored hazmat suit and begin to survey the wreckage around you, collecting now-legendary moments along the way.

A pipe crumbles before you, revealing your first – but not last - headcrab. You duck under errant lasers and find a crowbar, using it to smash through a glass door. You team up with a security guard named Barney, beset by zombies. Blue arcs of electricity sizzle through a computer room that explodes as you pass through it. You watch helplessly through a window as a scientist is attacked by a headcrab. Down the hall, a flickering computer screen illuminates a second man, shuddering and writhing as a mounted headcrab zombifies him.

Whether they were non-interactive moments that heightened the atmosphere or very interactive moments that kept the player constantly on edge, most of these elements were new to the gaming world then. And first-person shooters haven’t been the same since.