There was a time in my life, about ten years ago, when my craving for a great spicy chicken sandwich suddenly became too big to ignore. Thus, assuming my body knew what it was talking about, I set out to find the best spicy chicken sandwich I could.
As fate would have it, at that time just about every fast food restaurant had a spicy chicken sandwich to offer.  At the time I was working at a place that was in Atlanta and generally near several big business areas, so I had no problem getting to most of my options. To fulfill my quest I then set out to methodically visit every restaurant that was offering what could potentially be my culinary holy grail.
Here then are the results of that endeavor. Granted it has been quite a few years and I may have forgotten some of the finer details, but the final results are, I assure you, accurate. 
Wendy's Spicy Chicken Sandwich: I started with Wendy's because I thought this could possibly be the beginning and the ending of my quest. After all, the commercials at the time showed people eating the sandwich and melting hockey rinks! I made my first pilgrimage to the house of Dave. 
The chicken itself was of good thickness and looked nearly as good as the advertisements. Much to my surprise Wendy's offering was anything but acceptable. The sandwich had some spice to it, so you could taste a flavor difference, but it had little to no actual heat to it. It was a fine piece of chicken, but it was about as spicy as my grandmother. I was completely let down.
Checkers Spicy Chicken Sandwich: There wasn't a Checkers near my job, so one day I traveled to a suitable Checkers that was about halfway between work and home. Like most of Checkers food the sandwich was cheap, so I bought two.
It comes on a standard Checkers' bun which was fine. There was the smallest dab of mayo on both sandwiches. Just enough that when you bit the bite that would contain the exact center of the sandwich you might notice it.  And there were a couple shreds of lettuce put there almost as an afterthought. The chicken itself was a cheap thin patty with sad breading on the outside.
Unfortunately, there was again no spicy. In fact, both of the sandwiches were so not spicy I came to the conclusion that they must have gotten my order wrong and given me plain chicken sandwiches. For those of you unaware of how Checkers works, it is essentially a drive through only, so by the time all of this was discovered, I was miles away.
I was in fact so convinced that this must be the case, that I went back a few days later and ordered two more spicy chicken sandwiches. This time being sure to carefully enunciate and confirm my order. Sadly, the comestibles I got were just as completely unspicy as the last pair. Complete failure.
KFC's Zinger Chicken Strip Sandwich: KFC had recently come out with a range of chicken sandwiches. They were not actually chicken patties, but instead they would put three of their chicken strips on the bun and dress them up in different ways. Included in this series was a sandwich with spicy zinger sauce on it.
The sandwich itself was actually one of the better chicken sandwiches you could get at the time. It had a good bun, lots of lettuce and nice piece of tomato. And because it had three chicken strips instead of just one patty, it meant you got more crunchy coating in the middle bites than you typically would have.
The zinger sauce was also quite tasty. It also had good spice to it. Unfortunately, because the spiciness wasn't actually part of the chicken itself, but instead a sauce applied on top of the lettuce by whoever made the sandwich, the amount of spiciness you got was very random. Even if you asked for extra zinger sauce  it still meant it was very easy to have many bites without spice.
Burger King's Spicy Chicken Sandwich: My memory is failing me on this one. Either Burger King had a spicy chicken sandwich for a limited time and I failed to purchase one in the appropriate time window or they never actually had one at all. Either way, it means the King let me down.
McDonald's Spicy Chicken Sandwich: Fortunately, McDonald's also had a spicy chicken sandwich that they were offering for a limited time. They had them on their dollar menu and they sometimes had them on sale for two for $1. As I was on a pretty tight budget back then, thus this was already very appealing. However, McD's isn't really known for their culinary prowess, so I didn't get them expecting very much.
The bun was no surprise and the typical McDonald's offering. It too had the amazingly small dollop of mayo and the sad lettuce just as the Checker's sandwich. Also like the Checker's sandwich the chicken itself was a thin and covered in breading. It was there the similarities ended though. Because the McDonald's sandwich was amazingly spicy. Not so spicy you couldn't eat it, but way more spicy than you had any reason to expect from a restaurant that typically caters to small children who's biggest adventure in eating is mustard.
I was right and properly gobsmacked. I consumed my two sandwiches in happy bliss and when finished I reveled in the slight after burn. They didn't look like much, but the McDonald's sandwiches were by far and away the clear winners. My quest ended in an unlikely spot with well known gold and I returned there many times in the following weeks. I must admit I was truly sad when the promotion ended and the spicy goodness could no longer be had.
In truth, the reason for my writing this post is that I happened to be at McDonald's the other day and the spicy chicken sandwiches are back. They are again a dollar each, but I don't believe that they had lettuce on them this time. In fact, it's making me doubt that there was lettuce last time either, but it doesn't matter. I wasn't eating them for the green stuff. However, they also were not quite as good as my memory tells me they were in the past, but that could in a large part be because of a later entry to the field.
Chik-Fil-A Spicy Chicken Sandwich: At the time of my original quest, Chik-Fil-A didn't have a spicy sandwich and thus was a non-contender. However since that time they have joined the fray and they brought a heavy-weight. Their spicy chicken sandwich is amazingly good. It has all the plump chickeny goodness of the regular Chik-Fil-A sandwich, but it is coated in mouth burning awesomeness. This is now my go-to sandwich for when I have a hankering and in truth I no longer even order their regular sandwich at all.
I do have to say that when they first came out I think that they were actually spicier than they are now, because the first several times I had them I could hardly eat them. I was truly amazed by how strong the spiciness was. In fact, the first day they were offered the Pook and I both got one.  She couldn't even finish hers and has never ordered another. Now-a-days they are still plenty spicy, but I don't find them nearly so powerful. Of course, that might just be my getting used to them, but I really don't think so.
You can also get them with pepper-jack cheese on them, but I've never been a big fan of that cheese. I always find it to be a lot of spicy and no real flavor. So, I've never actually gotten one. I prefer mine with a packet of their honey roasted bar-b-cue sauce on it. It's glorious.
Wendy's Spicy Chicken Sandwich: D+
Checkers Spicy Chicken Sandwich: F-
KFC's Zinger Chicken Strip Sandwich: B-
Burger King's Spicy Chicken Sandwich: n/a
McDonald's Spicy Chicken Sandwich: A-
Chik-Fil-A Spicy Chicken Sandwich: A+
ENDNOTE: If anyone should know of any other contenders, I'm always looking for something new. Assuming it's a restaurant I can get to without too much hassle, I'll be glad to give it a shot.
 - I'm not sure if this is pure coincidence or if all of these restaurants knew of my upcoming yearnings and thus all provided me with their offerings. Of course the more cynical minded of you might think that it was I who had either knowingly or unknowingly succumbed to the power of advertising. But that's just silly.
 - To within an acceptable deviation at any rate.
 - Thomas that is.
 - But probably not.
 - A request that was not always allowed.
 - en arches.
 - There was a promotion and if you made a "reservation" online you could get a sandwich for free.