I'm always on the look-out for good games with great boss battles... so... What are your favorite boss battles?

Depends on what type of boss battles:

Intellectual: TNO from Planescape: Torment or the Master from Fallout

Skill-based: any number of bosses from the Dark Souls series, probably Fume Knight

Impressive: the first Titan battle from Castlevania: Lords of Shadow

Scariest: Aaron from Undying

Funniest: the Joker from Arkham Asylum

Saddest: Sif from Dark Souls

Story-based: Irenicus from Baldur's Gate 2

Unexpected: the Outsider from Dishonored 2: Death of the Outsider

Wholesome: Grandpa from Stardew Valley

Esoteric: the plague from Pathologic (2)

Taxes

Do boss battles include "final missions"?

Because then I say the last mission if you play NOD from Command and Conquer 1.

Everything about this mission was absolutely amazing in terms of constructing suspense but also cinematic execution (also the ending music is amazing), the general payoff of the final mission is much better than any other "apex" of any other game I've seen. Whats even more amazing is that this game is from 1995, still haven't seen a ending like this even if we consider sequels or games with similar production values from its time. The other Command and Conquer games don't even come close to the ending of the first game, which is maybe one of the reasons why Command and Conquer itself as a franchise kind of became lost in time.

If you only consider "Boss Monsters" then I'd say fighting the Toronto core (NED) in Albion. Though its more about the anticipation and stuff happening in your head rather than the boss battle itself, which is extremly lackluster.

Shadow of the Colossus.

If we also include "boss" battles, that is, a final battle without necessarily a boss-like enemy present, then without a doubt:

Hitman Blood Money : Final Fight (spoiler warning - best experienced when you don't know it's coming)
Xenonauts : made a big post about the incredible final battle
Warcraft 3: Battle for Mount Hyjal is always a treat, if a tense one to round up an incredible rts campaign.

As for more traditional boss battles

Broforce has several that are a lot of fun, especially the last one against the devil
The Lost Vikings has a great final boss, where you need to send each of your 3 vikings along their own difficult routes to get to positions from where they can take out the boss together.
Darkwood has a final boss that is incredible to face for the first time, though it's a bit of a hidden ending. It's one game in which it really pays to pay close attention to your environment.
Tiberian Sun : Firestorm the final level, at least as GDI has an actual final boss. An AI that activates and takes command of a massive mech.
Batman Arkham City's Mr Freeze boss battle is incredible
Commander Keen 1 is I think the first boss I ever fought, and he left a lasting impression. Not much of a fight really. You just shoot a chain holding a big block in place above his, office?

- HIND in Metal Gear Solid

- One of the Bhaalspawn in Baldur's Gate 2, the one in the ice cave

- The Fury in Metal Gear Solid 3 (yes, The End is cool too)

- The Master in Fallout

I like Dragon Warrior 4's final boss. Basically, it's a 7 (IIRC) phase boss fight where the boss starts out moderately strong, gets weaker, and then gets stronger to the point of being very strong at the very end. This battle is made more interesting by the fact that you can switch out party members mid-battle, but can't easily restore MP mid-battle (though I think the (mandatory) ally AI is smart enough to use MP restoring items that you put in their inventory ahead of time). Essentially, this battle encourages you to manage your resources throughout the fight, sending out characters who are generally less useful (like Brey (Borya)) or who aren't MP intensive (Ragnar, Alena, and Talloon (Torneko) come to mind here).

Final Fantasy 5 also has some interesting boss fights, some of which can be seen as puzzle bosses. The DS remake of Final Fantasy 4 is also like this. (One major difference, however, is that FF4 bosses tend to be immune to all status ailments, whereas that's not the case in FF5.) It's rather unfortunate that the boss fights of later FF games are not as good (well, FF10 has one interesting puzzle boss, but it's marred by the fact that you're likely to wipe in the third phase of the battle on your first attempt and there's an unskipable 5 minute cutscene that you have to watch before each attempt). Final Fantasy 9's superboss fight is fun to watch (the fight is very RNG heavy; you just need to hope he doesn't hit you with Curse (heavy damage + status effect) or meteor (something like 199-9999 damage to your whole party; I have seen it kill everyone but the character who was almost dad), but might not be so fun to actually attempt; FF10's superbosses, particularly the ones introduced with the PAL/International versions, are trash (the No Sphere Grid challenge is far more interesting).

The fight against Electrocutioner in Batman: Arkham Origins.

Hmm... it would probably be Monsoon and Armstrong from Metal Gear Rising Revengeance.

Can't remember any other boss battle I found extremely interesting. Perhaps Dante in DMC 4.

The final boss from Serious Sam (running from a 330ft monster towards an entire army and the Great Pyramid in front of you).

The stage 3 ship from R-type (which was basically the entire stage).

Helya from World of Warcraft (fighting her on a ship as she smashes it apart).

Helya from Hellblade (the first time you meet her, not sure it really counts but ...).

Rampager from Borderlands 3 (he smashes the place up too).

Several of the ships from Xenon 2 (some bigger than the screen).

Giant Walrus from Gemini Wing (fighting it in a waterfall).

Most of the bosses from Super Probotector ('nuff said)

Final boss from first Prince of Persia (you'll know what I mean if you've played it).

Ship/Woman boss from Resident Evil 5.

Boat boss from Resident Evil 4.

Final boss from Resident Evil 7 (although I'm not a fan of 'unkillable' enemies that you have to avoid or stop a million times).

First boss of SWIV (still my favourite vertical Shoot 'em up').

Final boss of Serious Sam 3 (giant bastard fighting another giant bastard in an arena and sky full of bastards).

Giant space ship also from Serious Sam 3.

Lucifer from Ghouls and Ghosts (and every other boss in the entire game).

The Cube boss from Legend of Grimrock (funniest boss).

Dead Space 1 and 2 final bosses (poor Issac doesn't have much luck. By the way the 'intro' level of DS2 must rank up there with the all time greats).

Scarecrow from Batman Arkham Asylum (Jesus Christ).

Dragons from Skyrim.

Shodan from System Shock 2.

I don't usually like boss battles. They're often tedious.

The only game where the bosses didn't irritate me was Broforce. Since every time you die you change to another character with different abilities, it keeps battles fresh.

Interestingly, bosses from the earlier levels often end up becoming "regular" enemies in later levels.

Also, I don't always have to face the bosses head on and can attack them in other ways. For example, in a later boss battle you fight a giant alien worm. Attacking from the front will get you killed almost every time, but there's a way to get inside the enemy without getting hurt and then killing it from the inside.

Imlerith and Eredin from The Witcher 3 are definitely up there. Also, it's nothing special but I still love killing the King Black Dragon from RuneScape, probably because it was my first MMO boss ever.

SaGa 2 has some interesting boss fights later on. There's one, for example, where you can't do meaningful damage for the first 5 rounds, then have to deal with a barrage of attacks (hitting your whole party every other round, and someone will die on the second round of this if you don't heal), and with three different ways the battle can end with the player's victory. It's also interesting that it comes in a game that hasn't previously emphasized healing during boss fights, but not you *have* to heal or you won't survive. (Fortunately, the game does clue you in to this; your temporary party member at this point in the game comes with a staff that heals your party (and you can buy more in many item shops, including one in the town that's accessible at this point), and the previous boss fight likes to attack your whole party, so you'll likely be using said healing there; also, there's another healing option that you have just for this fight.)

Interestingly enough, the strategy I've found to make this boss reliable with almost any party (as long as at least one of your regular party members has at least 612 HP) has to take into account an integer overflow bug that was fixed in the DS remake. (The bug makes it a bit harder than it otherwise would be.)

Icon of Sin - Doom 2: an unorthodox take on a boss battle.

Piggsy - Manhunt: Games usually don't scare me, but this boss fight was pretty intense.

Torrasque - Starcraft Broodwar: Pretty intense mission. In addition to repeatedly respawning until the cerebrate who controls it is subjugated, you are put under pressure from addtional zerg attacks.

Brock - Pokemon Red: Easy enough for me now, but the first time I played it I had a really hard time. Choosing charmander as my starter didn't help at all.