alright im gunna attempt to break it down.. my top 3.
salamence is probably the most THREATENING DDer, imo. the main reason being is because it outspeeds genesect, which is pretty much the DD killer. 100 speed means that you will be able to dispose of genesect quickly with a fire blast, and it can also net you a moxie boost. like others have mentioned above, once moxie gets going, it becomes pretty hard to stop. moxie + that ONE extra speed point over genesect is the reason it is the most deadly. i am fully aware that genesect is not the ONLY scarfer capable of revenge killing mence, but at the moment, it is the most common and the one scarfer dominating the metagame in terms of usage.
now, in order for salamence to sweep, there a few key things needed to be done. first off, removing steel types. this is easier said than done. back in the day, we had magnezone to stop ferrothorn and forry from running rampant. but in the current metagame magnezone is pretty ineffective. forretress can volt switch, scizor and genesect can u-turn, heatran will be trapped but can dispose of zone easily. this means that ferrothorn, skarmory, and jirachi are the main ones which zone will be trapping. however, ferrothorn and jirachi are usually teamed up with a politoed, to remove the 4x weakness for the former and usually to aid the subCMer in the latter. this becomes difficult for specs zone to kill these pokes off. another omnipresent threat is PRIORITY. being 4x weak to ice shard is one of the few reasons why salamence wont sweep outright. if your opponents team is lacking an ice sharder, well that just makes sweeping ten times easier. however, ice shard is not the only of salamences worries. although it resists mach punch and aqua jet, bullet punch is a big thorn in mences side. add in the fact that it is stripped of 25% of its health every time it switches in due to SR, and you will realize that killing off mence is not TOO difficult.
dragonite is the second most threatening. although it has more natural bulk than most of the other DDers, it also shares that nasty 4x weakness to ice with mence. although it has multiscale, we cannot always assume that SR will be off the field. this is actually one of the reasons that salamence is given the nod over dnite for most threatening. dragonite (especially bulky DD versions) not only needs its counters removed, it also requires a rapid spinner for dragonite to function at its fullest. now, its pretty obvious that every dragon dancer requires some kind of support, but the fact that dragonite needs its team mates to get rid of walls AND to keep SR off the field means it will have a harder time setting up. also, genesect exists, meaning even a +1 dragonite, assuming its not full health, will lose against the insect.
finally, i think gyarados is tied with dragonite in terms of being a threatening sweeper. first of all, with rain being the most used weather, gyarados can smack things around with rain boosted +1 waterfalls. that in itself is highly effective. as numerous people have mentioned above, it isn't weak to any priority move which makes it a lot harder to revenge kill. and to top it all off, it has two useful abilities in moxie and intimidate. intimidate makes it easier to set up on physical mons, but for a more offensive route people will find moxie to be the preferred choice. in the same vein as salamence, it only needs to kill off 1 or 2 pokemon in order to snowball and eventually wipe out the entire team. however, unlike salamence, the standard DD version is easily revenge killed by genesect. however, variants of gyara with bounce can dispose of gensect assuming it isnt carrying thunder.
just my 2 cents
