I just want to know which I should be running on a D-Dance set.
I'd say Jolly as Most Dragon-types have enough attack already and scarfers are quite common. Depends though. I listed the preferences below.
Charizard X: Jolly because Attack stat is high enough. Gyarados: Once again Jolly as attack stat is high enough and decent defencive stats allow it to set up more. Dragonite: Jolly. This thing is likely to get outsped otherwise. Feraligatr: Jolly because 109 base Attack is enough and speed (base 76) is not great. Tyranitar: 134 base Attack is monstrous as it is so Jolly as it has crappy speed. Crawdunt: Jolly. This thing is going to get outsped anyways but there is less chance with a Jolly nature. Salamence: Jolly. 135 base attack is massive and 100 base speed is average so Jolly. Scrafty: Jolly again. Speed issues. This thing is slow as hell with 58 base speed. Haxorus: Jolly. Attack is good and speed is not great. Tyrantrum: Jolly. 71 base speed is not good at all and 119 base defence lets it survive a physical attack.
Hope I helped! -It's a secret!
Simply for the reasons that at +1 Speed, you'll already be outspeeding the majority of whatever you need to outspeed. However, if your Pokemon is lacking the speed it needs (or you simply want to outspeed a particular threat) Jolly is a viable choice.
Hope I helped!