Left 4 Dead 2 represents the peak of co-op design. While not perfect in every way, L4D2 takes what made the first great and enhances it.

The gameplay has matured, and evolved (oh the irony) in key ways. Level design is the primary evolution I took note of when reflecting on my experiences. One level has you running across a roller-coaster in a frantic panic; all while another episode requires you to face the horde within a hurricane and flooding! Each episode has a gradual level of escalation that seemed somewhat absent in the first game.

Episodes like Dead Harvest and No Mercy often felt like they were missing something, and I believe it's due to the perspective the sequel grants. Whereas in the first level of L4D has you run through streets and to the top of a building, the sequel has you descend a burning building with the new mechanics, and capping off with a mad scramble for gas cans.

Left 4 Dead 2's weapons add a new layer to the gameplay that compliment the original's design. Melee weapons, magnum pistols, greater weapon variety, new throwables, new special infected and uncommon infected to make good use of these weapons. Falling in love with this game typically happens at the point you pelt a boomer with its own bile. Sitting back and observing a hoarder beating the living hell out him until he (or she) gets popped like a zit!

I could go on, but this is a very old game. I can only hope that Valve stops pulling a Nintendo and sitting on this series. If they refuse to make sequels, they need to release the license to someone else! Even the studio behind the first, and Evolve, Turtle Rock Studios, is making a spiritual sequel. There's a market for this series, and it's a damn shame that Valve is not providing a sequel. I would take Left 4 Dead 3 over Alyx or that failed card game.

For fun, let's rank some levels!
~Worst~
The last stand - Not a campaign, but it's a fun map.

Crash Course - Lazy and too short.

Cold Stream - A fun, but somewhat forgettable fan-made level.

The Passing -A really solid level that has a unique perspective

No mercy - This is where the list really gets going. This should be everyone's first L4D level. It's a fantastic start with a memorable finale. That said, it's only lower by default as the series gets better.

Death Toll - A fun, simple, and really charming level

Swamp Fever - The Weakest level from L4D2 base game is still amazing. It's one of the scariest levels to boot.

Dead Center - The introduction to the sequel is very similar to the first game, but more refined and really solid.

Dead Air - A very cinematic and clean cut level that never gets old.

Blood Harvest - This is surprising. The finale, the level of detail, the setting, and the intensity of this level makes it the best original map!

The Parish - With one of the most iconic finales in the franchise, a beautiful southern setting, and an amazing use of level design, the Parish is easily one of the best.

Hard Rain - This used to be my favorite level, but as I have grown up I see its shortcomings, but man it's still good. The rain's intensity, and the insane amount of water and debris is too gripping to overlook. This is the level that feels like a genuine struggle to survive. With the right amount of pacing, unique enemy encounters, the coolest use of backtracking and escalation I have ever seen in this series, Hard Rain still stands out as one of the best!

Dark Carnival - MIDNIGHT RIIIIIIIIIIIIIIDERS! This is the level everyone remembers. From the build up to the carnival itself, a lawn gnome escort side quest, a roller coaster horde section, and a grande finale with fireworks and dirty, southern rock, this level tops the charts. This is perfection from start to finish and makes great use of Carnival game tropes like tunnels of love and merry-go-rounds. If you haven't played Left 4 Dead 2's Dark Carnival, it's worth getting some friends together to experience the best this series has to offer.
~Best~