In your Opinion, What is the Hardest achievement to get in Left 4 Dead 2?

[Pick]

I help people get the "Ghost of Christmas Present" achievement from time-to-time, and some people have told me that it's Harder to get than"Still Something to Prove", "Supreme Survivalist" and "Killing Spree" Combined. Looking at the Global achievements, it seems the Hardest is a 3-way tie between having medal on every survival map (Bronze, Silver, Gold Respectively). In Second Place,"The Big Ten" is the Runner up; Granted does require a Versus Survival lobby (and i am pretty sure those are about as extinct as the Dinosaurs) but I do see them pop up from sometimes, so not super rare, just normal rare. Plus people are constantly organizing lobbies for those in the Forums, so not rare if you actively go looking for it. In 3rd place is "The Last Frontier", so basically an Expert Realism campaign of TLS, currently i have been trying to do this solo (no bots, only me) and it's a slog for sure, but compared it other Expert Realism Campaigns (Swamp Fever and Hard Rain are really good examples) it's a cake walk, basically a stroll in the park! Me, personally, I think that "Shotgun Wedding" is the Hardest. it depends solely on the player having the aim of a living God. the room for error on crowning (killing a witch with a single shot to the dome from crowning) is basically a pin-head sized box.

TLDR; People and Stats can't agree on the Hardest achievement for the game

[Geebanger0]

The one where you need to get a medal in survival mode using only melee weapons. Not only is that a difficult feat in and of itself, but its also possible for the achievement to become locked if you already have a medal on every survival map, making it impossible to achieve.

[ant]

gaurdin gnome

[JAiZ]

It's a bit difficult to compare achievement rarity, because the difficulty will change based on whether you're solo or in a team, and not all achievements existed at the same time.
For example, while it may be easy to obtain THE LAST DASH, that achievement has only been out for 3 years, compared to something like PORT OF SCAVENGE, which has been out for over a decade:
Although in total 2x more people have played 5 rounds of scavenge, that achievement has also existed for more than 3x as long, so when you adjust for that, there's more people who beat The Last Stand in under 13 minutes each day than people who played Scavenge for 5 rounds each day.
Even a trivial achievement like GET SKEETED ON (Kill a pouncing Hunter with a shotgun), which is as easy as ACID REFLEX (Kill a Spitter before she is able to spit) has a lower unlock rate purely because it hasn't existed for as long.

Then there's also the fact that some achievements may be insanely difficult for a new player, but really easy for an experienced player; Do a lot of people struggle with an achievement because it is highly complex, or because they keep running around like headless chicken?
Some players may find KILLING SPREE (Earn a gold medal on every official survival map) difficult, but it's not mechanically challenging due to how easy the first 12 minutes of survival rounds are, so it's only time-consuming, but then Z-GENOCIDEST 2: EPISODE 2 (Kill 53,599 infected) takes considerably longer to unlock, so technically that would be the hardest by this criteria - but then again, neither achievement requires any notable level of skill, they just require time.

SHOTGUN WEDDING (Kill the Witch bride with a single shotgun blast on expert difficulty) is also not very difficult, since all that's needed is basic game knowledge; Witches take reduced damage based on distance, so if you crouch up against the witch and land a headshot, it's hard to fail the cr0wn.

WEDDING CRASHER (As the Charger, grab a Survivor and crash them through 8 chairs at the wedding) is very difficult, in the sense that you will almost never see a survivor in the right position to charge them for the achievement, so it's only "difficult" because the odds of everything aligning perfectly are extremely low, not because it requires exceptional skill or game knowledge.
This is similar to CONNECTING FIGHTS (Play a Versus game on Dead Air from start to finish), as you'll pretty much never get a full Versus lobby where nobody ends up leaving before the end, so it's not difficult to achieve, it's just reliant on factors outside of your control.

So what exactly is the "hardest" achievement?
If we go by rarity, GOOD GUY NICK (Plays games with free weekend players and helps them survive a campaign) is by far the hardest, because it has only been obtainable for about 0.27% of the game's lifespan.

If we go by unlock rate, it's PORT OF SCAVENGE (Play 5 full games of Scavenge on The Port), WEDDING CRASHER (As the Charger, grab a Survivor and crash them through 8 chairs at the wedding), and SACRIFIZZLE (As a Special Infected, incap someone who is trying to sacrifice themselves), as they have the lowest unlocks-per-day ratio.

If we go by total time required, it's Z-GENOCIDEST 2: EPISODE 2 (Kill 53,599 infected) due to it taking the longest to complete, taking around 180 hours (7.5 days) of gameplay, assuming the average rate of 300 kills per hour.

If we go by how feasible it is to unlock an achievement on your own, then achievements like CHAOS GENERATOR (Have all 3 generators running at once in "The Sacrifice" finale), GHOST OF CHRISTMAS PRESENT (Spread cheer by helping a free holiday player survive a campaign), and CONNECTING FIGHTS (Play a Versus game on Dead Air from start to finish) - and any other multiplayer reliant achievements - are completely impossible to obtain without the help of other people.

If we go by raw, mechanical skill required, it's probably THE REAL DEAL (Survive a campaign on Expert skill with Realism mode enabled), STILL SOMETHING TO PROVE (Survive all campaigns on Expert), THE LAST FRONTIER (Survive The Last Stand campaign on Expert Realism) or ONE HIT WONDER (As the Hunter, land a 25 damage pounce), due to the fact these require a certain amount of game experience to successfully pull off.

Not to mention that some achievements have broken or unspecified criteria that may make unlocking them significantly harder, like BEAT THE RUSH (In a Survival round, get a medal only using melee weapons) which has not been tracking properly for a while now, or WING AND A PRAYER (Defend yourself at the crashed airliner without taking damage) which omits important criteria that may unknowingly void attempts at unlocking it.

Ultimately, it's hard to define one specific achievement as "the hardest" because of how many things there are to consider, and it gets even more convoluted when trying to account for people's individual, subjective opinions; One player may breeze through an achievement that an other player finds impossible to achieve, even if they are on the same skill level, just because they took interest in a different gamemode or playstyle, or got lucky.

[supersand]

Based on achievement percentages, The Last Dash. It isn't even hard its just RNG based on if the tank spawns too far away on the finale.