2008 was a blockbuster year where it really was just one amazing mission after another.
Also the year I first started playing FMs (other than T2X).
Two of my three favourite FMs of all time were released this year: Broken Triad and A Night in Rocksbourg 3: Ink and Dust. The other being The Seven Sisters released late the previous year.
Both Broken Triad and Rocksbourg 3 are polished to a mirror shine in just about every way. One has to work very hard to even find nitpicks.
Rocksbourg 3 has a beautiful and darkly mysterious atmosphere, intriguing backstory, a mix of traditional Thief and puzzley gameplay, and some of the best voice work to ever grace an FM. It's also one of the best looking FMs that OldDark has to offer. By 2008, visuals were really starting to be pushed to new heights, and this is an excellent example.
Broken Triad may be most immediately memorable for its gorgeous second mission, but the whole thing is brilliant in every respect. The seemingly simple premise (a museum heist) gradually opens into a complex plot full of unexpected twists, with excellent storytelling tinged with mystery and sadness. The mission's strength really is the story, despite everything else being top-notch as well. The audio is also well-chosen and adds to the melancholic feel.
Deceptive Perception 2 takes the concept of the original (very short but sweet) Deceptive Perception and expands it into a horror rollercoaster where you never quite know what's real and what's going to happen next. It keeps you on edge and makes you feel like you might be going insane right alongside Garrett.
Memoirs of a Dead God is tricky to talk about without spoiling it, but suffice to say that from the briefing it seems like it might be a Casing/Masks sort of situation, but the second mission is not what you expect.
Pirates Ahoy!, well, it starts off with a solid city mission and then moves to a series of very pretty tropical islands (with snakes, but we can forgive that, right?). It's a type of locale that's pretty rare in FMs, so it's cool to take a tropical vacation on a pirate ship (or multiple!) and see the sights.
I also voted for Mission X, because while the gameplay is not my cup of tea, I do appreciate its high production value and impressive technical achievements.