One important aspect of the final boss is individuality. The final encounter in the game has to be unique, whether by forcing you to use a combination of everything you have learned or acquired during the game, or by being nothing but simple, one-on-one combat with slick presentation.
The final boss need not be the hardest (just look at Final Fantasy), but it certainly shouldn't be a pushover. If the previous bosses in the game have conditioned you to expect challenging boss-fights it's going to be a real let-down if the last boss suddenly croaks with seemingly little effort.
Story is important: the player has to feel like they're going into a final confrontation, as if this is the end of the story proper, followed by the epilogue. Similarily, the boss has to make sense from a writing standpoint: if the apparent final boss has to be followed up by another, even more difficult encounter, then it should make sense (OoT, as stated before), and not simply appear out of nowhere (FFIX, I look at thee).
However, one of the most important aspects of a final boss battle, to me, is it's ability to stand apart from the consequences of previous gameplay as much as possible. I do not refer with this to level design or mood: a boss is supposed to have a connection of some sort with its surroundings. I think it best if I use an example instead.
[WARNING] THE FOLLOWING CONTAINS SPOILERS FOR NINJA GAIDEN FOR THE XBOX AND ALL FOLLOWING UPDATES
The second to last and last bosses in Ninja Gaiden are some of the best examples of hackneyed scripting and evil (yes, evil) continue systematics in modern gaming. The second to last boss is apparently the Holy Vigoor Emperor, a fact which didn't dawn for me until I played the game through for the fifth time. Besides the scripting, there isn't much that is blatantly wrong about it: it may be somewhat less thrilling than the other bosses in the game. But what Team Ninja really bungled is the aftermath. The player starts off next to a save point, which in Ninja Gaiden is effectively a continue point. This continue point lies at the bottom of an irritating platforming sequence, platforming not being one of the game's strong suits, which means (in Sigma and possibly the original) that if you happen to die during the last boss, you have to do it all over again. An irritating and completely avoidable waste of time, but there's more. For some reason which escapes me, the designers felt that not including a shop next to the aforementioned save point would be conducive for a good challenge. This means that if you fought tooth and nail to defeat the Vigoor Emperor (who was no cakewalk) and subsequently ended up with, say, nothing but an Elixir of Spiritual Life in tow (the weakest portable healing item in the game), you would be hip-deep in excrement and sinking fast. This means that you are presented with the choice of either playing the last boss enough times to attain the skills of a master ninja so that the one Elixir is enough, or loading and playing through the Emperor again, this time trying to use less items. This happenstance, coupled with an addmittedly rash saving procedure on my part, led to me once being unable to finish the game after having toiled my way to the final boss: the lack of a shop meant the difficulty suddenly rose a couple of levels. This, coupled with one of the most ineptly scripted and badly implemented dramatic revelations in my personal experience led to me viewing the final boss an unsavory light, despite the fact that from a gameplay perspective, he is a very good boss.
SPOILERS END
On the whole, the final boss, if the game even has one, should be the end of the sentence (the grammatical one, smartypants). It should be a challenging and satisfying end to the game that stands slightly apart from the rest of the experience, so as to provide the player with a simple, pure conflict that somehow summarizes everything that has happened up to that point.
The final boss need not be the hardest (just look at Final Fantasy), but it certainly shouldn't be a pushover. If the previous bosses in the game have conditioned you to expect challenging boss-fights it's going to be a real let-down if the last boss suddenly croaks with seemingly little effort.
Story is important: the player has to feel like they're going into a final confrontation, as if this is the end of the story proper, followed by the epilogue. Similarily, the boss has to make sense from a writing standpoint: if the apparent final boss has to be followed up by another, even more difficult encounter, then it should make sense (OoT, as stated before), and not simply appear out of nowhere (FFIX, I look at thee).
However, one of the most important aspects of a final boss battle, to me, is it's ability to stand apart from the consequences of previous gameplay as much as possible. I do not refer with this to level design or mood: a boss is supposed to have a connection of some sort with its surroundings. I think it best if I use an example instead.
[WARNING] THE FOLLOWING CONTAINS SPOILERS FOR NINJA GAIDEN FOR THE XBOX AND ALL FOLLOWING UPDATES
The second to last and last bosses in Ninja Gaiden are some of the best examples of hackneyed scripting and evil (yes, evil) continue systematics in modern gaming. The second to last boss is apparently the Holy Vigoor Emperor, a fact which didn't dawn for me until I played the game through for the fifth time. Besides the scripting, there isn't much that is blatantly wrong about it: it may be somewhat less thrilling than the other bosses in the game. But what Team Ninja really bungled is the aftermath. The player starts off next to a save point, which in Ninja Gaiden is effectively a continue point. This continue point lies at the bottom of an irritating platforming sequence, platforming not being one of the game's strong suits, which means (in Sigma and possibly the original) that if you happen to die during the last boss, you have to do it all over again. An irritating and completely avoidable waste of time, but there's more. For some reason which escapes me, the designers felt that not including a shop next to the aforementioned save point would be conducive for a good challenge. This means that if you fought tooth and nail to defeat the Vigoor Emperor (who was no cakewalk) and subsequently ended up with, say, nothing but an Elixir of Spiritual Life in tow (the weakest portable healing item in the game), you would be hip-deep in excrement and sinking fast. This means that you are presented with the choice of either playing the last boss enough times to attain the skills of a master ninja so that the one Elixir is enough, or loading and playing through the Emperor again, this time trying to use less items. This happenstance, coupled with an addmittedly rash saving procedure on my part, led to me once being unable to finish the game after having toiled my way to the final boss: the lack of a shop meant the difficulty suddenly rose a couple of levels. This, coupled with one of the most ineptly scripted and badly implemented dramatic revelations in my personal experience led to me viewing the final boss an unsavory light, despite the fact that from a gameplay perspective, he is a very good boss.
SPOILERS END
On the whole, the final boss, if the game even has one, should be the end of the sentence (the grammatical one, smartypants). It should be a challenging and satisfying end to the game that stands slightly apart from the rest of the experience, so as to provide the player with a simple, pure conflict that somehow summarizes everything that has happened up to that point.