It doesn't have anything to match the first game's memorable twist, but in all other ways BioShock 2 is the best of the three BioShock games. It returned you to the underwater city of Rapture as a prototype Big Daddy, one of the drill-handed protectors you spend the first game hunting. It had better fights, better or equally as memorable world design, and never hits any of the low lows of its predecessor or sequel. (E.g. it doesn't end with you fighting a big blue man.) We rightly champion games which undertake the heavy work of creating something new, as the first BioShock did, but moment to moment, BioShock 2 is simply more fun.
I feel like I'm supposed to get fancy and start talking about how BioShock 2's exploration of collectivism is more nuanced than the original's objectivism, but it's not the skewering of political ideologies that makes my Big Daddy heart pump. I'm more interested in the game's rivet gun, which with its default ammo lets you skewer splicers to Rapture's damp walls. It's the closest the series ever came to giving you a bow and arrow, which is every game's best weapon. The rivets kill most enemies with a single headshot, and you can even pick them up again after they've done their work.
They also have an alternative 'trap' ammo, which lets you place them against walls like tripwires to further nail passers by. This is useful because the game follows a similar structure to the original, in which each area built towards the moment when you decided to take down the patrolling Big Daddy or Daddies. The problem with BioShock is that the Big Daddy fights could never live up to your expectations of them: they were like tense and difficult the first time or two, and trivially easy by the time you were pumped with plasmids or grinding them out via the vita chambers.
The Big Sister looks and acts like a more agile version of the Big Daddy, diving helmet included, but the fights against her are the best fights in the entire BioShock series. They are difficult every time, meaning you never stop being tense in the build up to them. You will want to prepare as much as possible, lay down rivets and other traps wherever you can, and being the lumbering hulk facing down a faster, hoppier opponent feels like a reversal of the first game.
If BioShock 2 was just better fights in some re-treaded locations, it'd still be a good if forgettable game. It's better than that though, and still worth revisiting today because a lot of its level design lives up to the first game. An early highlight for me is Ryan Amusements, which features animatronic Andrew Ryans introducing you to Rapture and its history. Except, of course, this is not the actual history, but the version Ryan wants its residents to believe. It's level design which delivers backstory, character and criticism all at the same time.
Another area explain the process by which Big Daddies are made, while the best of all is a brief section during which you play as a Little Sister. Little Sisters, it turns out, see the dirty and decaying world of Rapture as a gold and satin paradise. The dead are angels, flies are butterflies, blood smears are rose petals. Reality only snaps back in during the moments when you drive a syringe into a corpse's neck to suck out what's inside.
These moments and more are on a par with the first game's standouts like Fort Frolic. Maybe that's because BioShock 2's director was Jordan Thomas, the designer of the Fort Frolic (and of Thief 3's Cradle). Or maybe it's because they had so many decisions made for them by the first game that they could focus more on doing interesting things within the template laid down.
For my purposes, the why doesn't matter. I hear regular complaints about how many games released each year are sequels to other work, but BioShock 2 is a strong argument for every game to have a second go around.