Well Tyranitar really shouldn't be brought into this discussion, as Boondocker said, in a metagame filled with priority Tyranitar just isn't as good as it used to be.
Gyarados is indeed a decent DD user, however it also isn't anywhere near as threatening as it once was, although it still has more use than Tyranitar in this metagame.
Haxorus is indeed a good dancer, and when I run Haxorus I like to use the DD set, although he too is outclassed.
Now onto the real discussion, Dragonite vs Salamence.
Just so that its here for everyone to view I'll quickly post each of the two's base stats for convenience.
Dragonite
Inner Focus/Multiscale
- 91 HP
- 134 Attack
-
95 Defense
- 100 Special Attack
-
100 Special Defense
- 80 Speed
Salamence
Intimidate/Moxie
-
95 HP
-
135 Attack
- 80 Defense
-
110 Special Attack
- 80 Special Defense
-
100 Speed
*Bolded is the superior stat
By analyzing the stats side by side, it becomes quite obvious that Salamence has the superior offensive stats, with a slightly higher base attack and a substantially increased speed stat. Couple that with Moxie and Salamence is able to just keep stacking the boosts with each KO. That does make Mence the outright candidate for best DD user, however I wouldn't put the issue to bed on that alone.
What Dragonite loses in offensive prowess, it makes up for in its bulk. Dragonite boasts higher Defense and Special Defense, alongside the coveted Multiscale. What this means is that Dragonite, while never being a more offensive force than Mence, will generally be able to set up more easily. Mence will outspeed a lot more than Dragonite will, allowing it to possibly get that boost before the enemy attacks, and then responding with a boosted attack. Dragonite on the other hand, provided Multiscale is intact, will have a larger variety of situations in which it can set up. It is Dragonite's bulk which still keeps this discussion alive, as without it Mence would far outclass Dragonite as a DD user, however the bulk is there and it must be considered.
As of that, the two are on par because they each have their advantage. So what is is that can really separate and differentiate them? The answer: Not a lot. Because they share exactly the same typing, and very similar movepools, neither has a clear cut advantage over the other. What needs to be taken into consideration when choosing which one to use on a team, it generally helps to add it to the team in the latter parts of the team building process. This way you can establish exactly what sort of a DD Dragon you want, a bulkier one that has more situations in which to set up, or a more offensive force. Until one of them is granted a move that the other is not, in a future generation perhaps, they will always be incredibly similar. The choice comes down to what suits an individuals team better.
Personally I run Mence and Dragonite in equal amounts. I prefer to use Mence on my more offensively oriented teams, or stall teams that require one serious offensive threat. However when I do use Mence I am almost always using ScarfMoxie Mence, which would be Mence's most superior set by far. When I use Dragonite, its either the parashuffling set, or the DD set. Dragonite's most effective set is easily the DD set. The CB set is great to use, for immediate offensive power, although the DD set really does outclass that once you get a boost or two. By that, if you were to compare the two Dragons, and their best individual sets, you would have to say that Dragonite is the best DD user. Although I don't think you could say that and have it be true.
To reiterate what CrackinSkulls said, there is no definitive best user of Dragon Dance, although these two are at the head of the pack.
