The best items are choice scarf of life orb. Choice specs is an option, but the other two are preferred. Hydreigons speed is lacking, so improving that is key to success, but life orb increases damage output and allows you to switch moves.
Personally, I prefer scarf, but any of them is a great option too. It just depends on what your team needs and what you think is needed.
I also just saw an assault vest suggestion, but Hydreigon focuses on offense rather than defense. Also, an assault vest is not going to protect you from a STAB moonblast being yeeted your way by tinkerbell