For all passenger and freight cars and locos I do the following:
1) The couplers are always body mounted (NO talgo for me!) and they:
a) are 'dead on' in terms of height.
b) freely move from side to side (inside the coupler box).
c) are centered in the car and the coupler box is correctly aligned with the track (it doesn't run up or down either front to back or side to side).
2) I always use Kadee couplers. There are a few brands that I will 'tolerate' without changing out right away - but as soon as the coupler is even slightly suspected of creating a problem it gets changed out to a Kadee. I prefer the look of the #58 size Kadee's but prefer the operation of the #5s - the smaller heads just do not couple/uncouple as easily as good old #5s - and the 58s cause even more problems if they are coupled with a #5. If the car is expected to be switched regularly or run on "less than stellar" track that has problems with vertical alignment, vertical kinks, or just plain old 'humps', I will usually choose a #5 size head. The larger gather of the #5s often 'fixes' marginal track.
My basic rule is that if I am working on a piece of equipment for any reason - I change out the couplers to Kadee's.
I -never- run any of the all-plastic couplers (including but not limited to the infamous horn hooks) - not even for a test run. Simply because if they don't have an acceptable coupler it isn't a real test is it ... (see 1st paragraph above).
I often use the Kadee whisker couplers - but only if the coupler box prevents the coupler from drooping due to the lack of a copper spring. I have tried and been successful with just adding a copper spring to a whisker install - by first flattening out the side springs in the copper. I have also used a thin strip of styrene across the opening of the coupler box (glued to the cover) to prevent drooping.
3) I almost always use a Kadee coupler box. Especially on passenger cars because I am a huge fan of diaphragms and when they are installed you can't see the coupler box any more (reread the first paragraph above). I will use a different coupler box or even use the box supplied - but if I have a problem with uncoupling ... or even worse derailing ... I will change out the coupler box in a heart beat. It is amazing how often changing to a Kadee box will fix a problem that you didn't even suspect was related to the box!
I have experimented with the more prototypically sized coupler boxes (which are narrower) - and I love the look of them ... but also have found that on longer cars they cause derailments due to creating side forces on the car that are not there with a Kadee box.
4) If at all possible I avoid using an "offset" coupler to correct a coupler height problem. I will, sometimes, use an offset coupler to increase the height when using a regular coupler would result in serious removal of material from the model. When adjusting the coupler height my first approach is to adjust the height of the car where the truck meets the body bolster. But I'm not opposed to removing material to make a coupler sit at the right height/work correctly (see 1st paragraph!). If the coupler is too high - and the car is the right height off the rails - then I will add a styrene shim between the car and the coupler box rather than use an "offset down" coupler. Every time.
5) I prefer to mount the coupler box "on a flat plane of styrene that puts the coupler at the right height". I -greatly- prefer to use a single 2-56 screw into a threaded hole that goes thru the coupler box and I frequently cut off the 'ears' on the Kadee boxes. I almost always glue the 'top' (actually ends up on the bottom) of the Kadee box on the coupler box using just a touch of Tenax. About the only time I use any other method of attaching the coupler box is when it is a loco and has its own method.
6) If the bottom of the diaphragms interferes with the coupler I remove material from the diaphragms until they do not in a heart beat.
7) I will do #6 even on brass cars. I also have no qualms about bending/removing brass steam lines, brake hoses, or whatever is getting in the way of the operation of the couplers (see 1st paragraph). On freight cars I will often mount a brake hose that is "a bit too wide" in order to allow for full side-to-side movement of the coupler.
8) Although I have never done this, I have often thought that using all draw bars for passenger cars with diaphragms makes a lot of sense for a lot of reasons. I just need to try it. I will try this one day just not sure when. When I do it will be for one entire train of cars and I will probably use "PET" as the material for the draw bars. And they will be done in such a manner that the draw bar will be permanently held in one end and a post on the other car will fit thru a hole in the draw bar. The obs will get a 'scale' coupler (probably a functional #58).
9) Yes, I have experimented with most of the truly exotic solutions such as the Sargent couplers and I loved the way they looked. The operation was "acceptable" but I haven't even considered converting any of my fairly large fleet of cars to them. Will I ever change? Who knows. If they came out with some kind of "adapter" that made it easier to replace the Kadees, such as an insert that fits inside the Kadee box, then I might.
Given all of the above - my general advice about couplers is "if thy nose offends thee, cut it off".
Again - see the first paragraph. I wish that I had experience with -all- of the various coupler products that are available, but I don't (can't?). So my advice has to be 'generic' in nature. On the other hand I do have experience with all of the "main stream" products that are available for HO scale (and some for other scales).
I hope that I have "taught you how to fish" (*G*). At least I have provided my methods. There are other methods that also work ... these are mine.
In the end it REALLY does come down to -first- making the decision about whether you care more about the operation or the appearance and then doing what fits that decision.
Having said that - it is rare that you really have to compromise looks for operations.
- Jim