I have to admit, I didn't particularly care for the "theme" that these four seemed to shared. Twist at the end isn't a great short-story format. It's called forgetting to write a conclusion. It's cheap pulp (which can be a lot of fun), but doesn't rise to the caliber of Tales from the Crypt or even Goosebumps.

In the random order I read them in (again, keeping things constructive):

"The Mechanic" - Started real strong, but kind of lost it in the second half. If it had taken a more gradual or subtle approach, it might have been my first pick. Even with the way it went, what it really needed was a short wrap-up paragraph. Something to really hit home what changes occurred, possibly from a third person party (the woman who dropped off the car perhaps?). It was a close one for second place with this one and "A Live Arm," but this one lacked the bit of personality characterization the other had. edit: I think shinta pegged the issue pretty well.

"Jack the Green Slider" - I did not like this one the first time I read it. But after I read all the others, I went back to read this one again. Despite some serious issues with pacing around the middle, this one was the clear winner for me. It could use a little more clarity, but at least it had a legitimate ending to go along with its "twist," instead of a the surprise filling the place of an actual ending. It was amusing and had some humanity in it.

"Madness' Door" - Frankly, this one is a bit of a mess. It feels like some kind of Warhammer fanfic. It's too dense, lacks clarity, and doesn't provide sufficient setup. If it got thinned out a bit, and like "The Mechanic," added in a little conclusion paragraph instead of finishing with a "twist," it might have worked pretty well. That said, what it lacks in proper narrative, it makes up for in development. It has a good flow overall, has a nice feeling of madness to it, but paragraph per paragraph it needs some considerable patching up.

"A Live Arm" - I went for this as my second pick (alluded to above), because it had a fairly strong characterization of the narrator. It gave a good feeling of motivation for why the condition was accepted, had a good setup and a decent rising action sequence. However, it's lacking in an actual climax and again...a conclusion. It's the last several paragraphs that push it over "The Mechanic," there is a strong feeling of foreshadowing, a bit of a change in character as the narrator dismisses Steve. While I don't really care for the "twist," it has a strong foundation overall.