Which one was it for you? Most of those who've made it through to the end of Valve's brilliant sequel will have encountered at least one test chamber whose solution remained so unfathomably elusive as to induce genuine despair.
Few puzzle games more skilfully manage and manipulate the player's emotional experience, capable of transforming us from humbled dolt to crowing genius in the space of a couple of trigger pulls.
Like that other notorious cortex-molester Braid, the solution in Portal 2 is always staring you in the face. It doesn't half make you feel stupid until you work it out, but Valve's environmental cues are often subtle enough to encourage the belief that your escape ranks alongside the discovery of fire in the pantheon of human achievement.
In recognition of this we've picked out the ten chambers we found most challenging, memorable and just plain great during our single-player playthrough. And, for those of you yet to beat the game in its entirety or keen to relive the moment from another's perspective, we've also captured the solution to each.
In short, these are the puzzles that left us bewildered, frustrated and awe-struck in equal measure – and filled with admiration for Valve's fiendish ingenuity.
This, of course, only represents our experience. We'd love to know how it was for you, so please let us know your standout, synapse-busting moments below. Meanwhile, here, in reverse order of misery/joy is our top ten.
And, if you haven't played Portal 2 yet then, y'know, spoilers.
10) Chapter 6-3
The further you get in the game, the better you get at it. Looking back on this chamber after completion, it all seems so blindingly obvious. But that just shows the extent of the brain-rewiring process you've undergone.
This test provides the first stiff challenge using Repulsion Gel. The key is to grasp the difference momentum makes when landing on blue paint: it's the only way to make it across the long gap toward the exit. And at this stage in the game, it might take longer than is respectable to hit upon the solution.
9) Chapter 7-4
"If life gives you lemons, make lemonade". As the archived meanderings of former Aperture Science CEO Cave Johnson echo around the facility, the game serves up its first big Conversion Gel test.
The introduction of the new gel requires yet another rethink of your approach, calling for something straight out of the Jackson Pollock school of painting. And the more OCD-inclined gamer may find themselves (as we did), fastidiously whitewashing every last pixel before eyeing the solution high up in the towers.
"If life gives you lemons, don't make lemonade. Make life take the lemons back! Make life rue the day it thought it could give Cave Johnson lemons!"