Imo, adamant seems better if you are using mach punch. But as Orange stated, you gotta first check the tier and ask yourself some stuff.
On top of my mind:
- Do I want to outspeed non speedy nature breloom? Or Dragonite? Or metagross? These 3 are probs the most important to consider. Other stuff in the speed range can be hurt by mach punch or is inaccessible no matter the nature (There is modest kingdra too but more often than not those will be under rain so I didnt list it there)
Edit: For the second question, Im afraid I cant help much with haxorus, even tho I think jolly is preferred. For Hydreigon, I have used pretty much all possible variants and for an offensive one, you need to first know if you want hydra to wallbreak or to sweep. Both modest and timid are viable, but have different jobs.