Although it really depends in the Pokemon, I'm going to have to argue for Outrage
Outrage is the most powerful physical dragon type move, at 120 power, and is commonly used by physically oriented dragon types. It only faces competition from dragon claw, which is used for reliability, and backed the many powerful dragon types like salamence, haxorus and garchomp, it is very powerful, and it gets decent neutral coverage (only resisted by steel)
Petal Dance is an ok move, but there are more reliable options like Giga drain and energy ball. Also, grass is resisted by 7 types, so being locked into a grass type move is not really in one's favor.
Thrash is ok as well, but there are better normal type moves out there, most notably return, which almost everywhere Pokemon can learn, and double edge, which a good amount of Pokemon can learn. Also, body slam gives it decent competition for a slot. Normal type moves are also not to great to be locked into, as most rock and steel types can take them with ease and ghost types are completely immune to them.
Hope this helped