There are few things better than a good boss battle for the player to test their mettle against. But they don't always turn out that way. Batman: Arkham Asylum didn't figure out its boss fights until late in development, and it shows in its repeated Titan thug fights.
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Metal Gear has had much better luck with its. Its best fights used every aspect of its gameplay to innovate, like chucking rotten food to attack the boss' stamina, or making use of first-person view. But some are better designed than others, as these are the best Metal Gear bosses in the franchise.
The problem with the Beauty & The Beast Unit in Metal Gear Solid 4 is that they're retreads of MGS1's bosses. But Laughing Octopus was different because she was based on one who didn't get a boss fight: Decoy Octopus. He had one planned where he'd change into different guises, but the PS1 didn't have enough memory to make it work properly.
This idea was brought back for his shapely namesake on the PS3, with some new tricks along the way. She could blend in with her surroundings, lashing out at Snake if the player didn't catch her in time. Then she could disguise herself into objects and Snake's allies. If players didn't look carefully, they'd be done for.
Metal Gear Solid 5 was a low point in the series for boss fights. Quiet and Sahelanthropus were too similar to older, better bouts from the previous games. While its worst ones were usually mook rushes with the Skulls. This leaves the Man on Fire as its standout encounter, where it's still a good test of the player's skills. Bullets didn't affect him, and he'd block the exit if players tried to escape.
Instead, they have to use the environment against him to win. They can bring the water tower down on top of him to douse his flames, or knock him in the nearby pool with enough firepower (like planting C4 on the nearby propane tank). Or, flashiest of all, call in a jeep, lure him to the cliff edge, then drive the jeep into him to knock him off. Just make sure to jump out before it goes over the edge.
If MGS5 had too many mook fights, then Peace Walker had too many mechs. Nearly all the game's bosses were vehicles and the AI pod's different forms. The only non-mech bosses came from Monster Hunter's guest beasts. As interesting as they are, the titular Peace Walker is the most testing of the bunch.
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This four-legged monster has a running charge attack comes out of nowhere, an EM pulse that wards off all rockets until it's shot down, and plenty of S-mines and drill missiles. Even worse, Peace Walker can start its nuclear launch sequence every so often. If the player can't distract it with enough attacks, it'll fire and cause a game over.
The series has had fights against tanks and aircraft since the MSX days. They weren't particularly exciting though. The Hind-D in Metal Gear Solid 1 is the best of the helicopter fights, and even that bout can drag. So, in Metal Gear Solid 2, the developers stepped things with a VTOL. It still had to be shot down with enough Stinger missiles, but that was easier said than done.
It moved faster & further, and had harder hitting attacks with its bullets, missiles, and heat exhaust. It could even bombard the whole level, where the player's only escape was to get to the 2nd floor ASAP, or figure out Raiden's invincibility frames and time them just right.
Raiden's return in Metal Gear Rising: Revengeance called back to MGS2's themes of identity and memes (in the academic sense), though in a much lighter, action-based package with its own memes (in the internet sense). It also had a lot of great boss fights, with Jetstream Sam being the best, offering a straight-up, no frills fight.
He has more up his sleeve than just swift sword strikes. Sam can blast rocks at Raiden, race back and forth across the stage to attack from afar, and perform leaping attacks to close the distance. Even when he loses his sword, he can rush in and throw Raiden about like a ragdoll. He's a danger no matter how armed he is.
Metal Gear has a lot of Metal Gears, but few have appeared more than once. The few that have didn't come back as bosses either, except for Metal Gear RAY. This marine-based take on bipedal tanks made three appearances in the series as a boss.
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In MGS2, the player had to take down multiple models at a time. While in MGS4, they had to pilot REX to take it down in the series' only Gear-on-Gear fight. Then in MGR, it became iconic as the opening boss, where Raiden's parries opened it up for more punishment, set to the hard rock beats of "Rules of Nature".
That said, RAY isn't as iconic as its predecessor REX. Designed by Yoji Shinkawa for MGS1, the machine's chunky, angular look inspired subsequent Metal Gear designs. The Gekkos are just mini-REXs with RAY's legs. Peace Walker's ZEKE is REX with an AI gimmick, and MGS5's Sahelanthropus is REX combined with a Gundam.
MGS1 effectively built REX up as a threat. It has to be destroyed to save the world, but how? The strategy is simple enough: shoot missiles at its weak points, then chuck Chaff Grenades to keep its own homing missiles from targeting the player. But if they're not careful, REX's laser, machine guns, and heavy stomps can finish them off just as quickly.
REX is the best Metal Gear in the series, but it isn't the best boss fight. Not even in MGS1. It doesn't have the freak-out factor that Psycho Mantis brought to the table. This gangly, gasmask-wearing figure was weird enough on his own. Then he reads Snake's mind and memory card ("You like Castlevania, don't you?").
Once the actual fight begins, he'll read all the player's controller inputs too, basically becoming invincible. The only way to stop him was to break the 4th wall harder by moving the controller to Port 2, or smash his bust statues if they couldn't do that. It was a freaky encounter back in 1998, and is still strange and memorable 25+ years later.
Once the Metal Gears are finished, the series' final fights are often hand-to-hand affairs between the player and the big bad. Solid Snake went toe-to-toe with Big Boss in the first two Metal Gear games, then against Liquid Snake in MGS1. Raiden and Solidus brought the blades out in MGS2. Then MGS4 ended with an emotional fist fight between Old Snake and Ocelot.
Good as it was, it was done better in Metal Gear Solid 3. Naked Snake and The Boss cared for each other in a way beyond being war buddies or lovers, but they had no choice but to fight at the end. As a boss battle, it's fun and tests the player's CQC and camo skills. But its true strength is in its storytelling as the game's true climax.
But if The Boss is the best at storytelling, what's the best fight at gameplay? Some might contest MGS3 's The End being this high. His fight can be slow, and the game offers ways to get around it (e.g. setting the console's clock forward by a week). Yet it's also a unique fight because The End has to be taken down with the series' big selling point: stealth.
If players can find his location, sneak past his gaze, and catch him unaware, they'll be rewarded. Holding him up gives them his special camo, and taking him out non-lethally will provide his tranquilizer rifle. That's just the start of it too, as the player has more tools and tactics to take the old man on with than they realize. It's the peak of tactical espionage action.
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