Power lifting, particularly the deadlift.
Least complicated rules: Rock, paper, scissors. If that does not count as sports, carom billiard.
As for the most complicated rules, I don’t know, but I am surprised nobody has mentioned cricket yet. Maybe that is because I don’t know the rules and don’t understand it at all.
Any kind of event in which you have to hit a target - whether it be with an arrow, bullet, axe, or other object. You want to get as close to the center of the target as possible. Of course - the rules would define the objects used, and possibly the positioning of the shooter/thrower.
That’s true, but at the competitive level, you’d be surprised at the rules.
I was a competitive rifle shooter, competing at very high levels (think Olympic tryouts). In addition to having my rifle weighed at every match, and the trigger pull weighed at every match–both had to be within parameters for the given match–my clothing was always in question. How thick was my shirt? My shooting jacket? What kind of shoes or boots was I wearing? Were they within parameters?
It was more complicated than “closest to the center,” although that was the ultimate point of the contest. Somewhere, I still have my rulebook, which was maybe an eighth of an inch thick. That may not sound like much, but it contained a helluva lot more than “closest to the center.”
pro wrestling as any and all rules are optional
I just looked up the rules of curling, and it’s 66 pages. The last 20, or so, are about the way different tournaments are structured, and the means of qualifying for them.
These days, you are probably right.
And you are, too. I played bar-league level for years, and can’t really recall any controversy, except Eric, who routinely crossed the line.
I’m guessing not many fetishists in the upper levels, then? Or maybe all of them? 
Thanks for the comments, everyone. It’s interesting that even the “simple” sports, at the professional level, have a lot more rules than you’d think at first glance.
I think American football wins this hands down. The fact that they have 8 officials on the field, three in a replay booth upstairs, and a direct line to New York is a pretty good indication of the game’s complexity. I think the simplest game is bowling.
I heard an interview once with the head of software development for one of the F1 teams (I forget which). He said there are rules governing how much computer runtime they were allowed to consume when running simulations. Wrestlers don’t have to put up with that. 
Also, there are rules governing how much time F1 cars can spend in a wind tunnel. There are strict budgetary rules. In terms of design, a racing car is made of hundreds or thousands of components and I’d hazard a guess that every single one is subject to some kind of written rule.
The more I think about this, the more I realise just how complex the regulatory landscape is for any sport.
If anyone is interested (I mean, you shouldn’t be…life is far too short) Here’s a link to the F1 sporting regulations.
Cricket is fairly concise by comparison but still fairly weighty.
Here are the LAWS OF CRICKET (note, laws, not rules)
I think it’s fair to distinguish gameplay rules from administrative rules. NASCAR and F1 have, I’m sure, very detailed administrative rules, but from a gameplay perspective it’s fairly straightforward. They disallow some dangerous maneuvers, require certain things to be done under caution, but otherwise it’s not that hard to manage.
The 100M dash is basically - stay behind this line until the starter pistol goes off, stay within your lane, don’t touch anyone else, and get to this other line the fastest by running. Any other rules are simple administration.
American football OTOH, has gameplay rules that are ridiculous, where you can stand, who can be moving, who can touch the ball, it’s pages and pages of gameplay rules, and still has administrative rules on top of it all.
To someone who doesn’t know the rules, fleet sailboat racing is difficult to navigate. There are large buoys that define the course, but until the starting gun nobody knows the pattern being run.
Also, there are multiple heats all racing at once. The rules for who has right-of-way are very specific and used strategically can make it look like certain boats are making crazy decisions.
Gameplay rules and administrative rules are terms I was groping for in the OP. Thanks!
For the purposes of this thread, I’m interested in both. In fact, I’m really curious which sport has the loosest administrative rules in particular.
Plenty of sports are much simpler than bowling. I think that sprinting pretty much has to be the simplest. Let me tell you the rules from sprinting: “Run from this line to this other line 100 m away before anyone else does”. Can you tell me the rules of bowling, in the same level of detail, in a single sentence?
Even in bowling the pins must have specific dimensions and weight, and then be set in specific positions in the lane. Balls also have a weight limit, must be made of materials with a specific hardness, and must be regularly surfaced except for the finger holes. Lanes must be a specific length and width, with regulations on surface materials, flatness, and oiling.
If it’s a well regulated sport there will be a lot of details to consider.
It’s going to depend a lot on how “meta” you consider the rules of the game being. I have here in front of the me the latest edition of the US Chess Federation’s Official Rules of Chess. It’s a 275-page book. Clearly the proximate game rules are much smaller, but actually running official USCF events, adjudicating irregularities, etc., requires rules. (Although, the core game rules do take up about 80 pages.)
Rugby rules are convoluted and opaque resulting in many baffling decisions. And I say this as a former player and referee.
Roll the ball down the lane at the pins and knock down as many as you can. LOL
You also have to tell people how many throws they have each frame, how many frames there are, that they have to wait for the pinsetter, that balls in the gutter can’t bounce out again and count, how to score spares and strikes, where they can stand and where they can’t, what to do in the event of a foul…
It’s simple, but not quite as simple as a dash.