It's a preference. There is no real thing as the best opener, since the damage you do overall by the end of the fight is what matters the most. I personally prefer opening with a form shift, followed by a dragon kick to enable its effect right out of the gate though.

There's a particular opener you can do by level 60 for a heavy burst of damage, but priority wise, seeing reason to execute it over prioritizing anything else felt unimportant. I'm sure it's either posted somewhere though, or someone may post it here to help.