I love to do my Aerial Dankukyaku’s, I just have a few questions about it:
1.- Is the HK version safe?
2.- Does it have any priority?
3.- Are there any combos after hitting with it?
Thank you in advance.
1- I dont think any version is safe on landing.
2- Lk Dankuu wrecks pretty much anything in the air and a lot of anti airs
3- Only one i know of is a corner combo, koryuken fadc in the air ex dankuu ultra 2
LK Danku is the only one with any sort of priority, as mentioned above. The best use of the other aerial Dankus is as a surprise attack to catch someone farther away from you off-guard by suddenly flying into his face. It can be somewhat effective against runaway Seths/Dhalsims when used sparingly.
An air to air Danku of any type can lead to Haoh Gadoken if the opponent is knocked into the corner, as long as the last hit of the Danku connects. Timing of the Ultra will vary depending on where you land and how you hit the enemy, you might have all day to input the Ultra (and thus risk doing it early and whiffing) or you might have a tight window. The LK Danku into Ultra will do more damage than any of the others, because the multiple hits of MK/HK Danku will strip extra hits right off the Ultra.
Air EX Dankuu also seems to have good priority. Air MK/HK Dankuu are pretty crap and will often just get you a whole world of hurt 
Thanks again guys, good to know. I’ll stop using those MK and HK versions too much.
From what I can tell though, that initial part of the MK and HK Dankuu has a “knee” animation much like Akuma’s ground HK tatsu. That “knee” startup seems to have some sort of priority, so if you meet someone air to air and you’re on top of them it may at least trade.
Not sure though, I prefer to abuse air LK Dankuus, and as others have said, use it as a jump in if you think the other guy is gonna shoryuken you. It trades at worst, and you get the better end of the trade from what I’m aware of.
At worst it loses flat out, depends how late they DP (or what the DP is I suppose).
the best is doing a HK danku over Hakan’s lay-down on the ground ULTRA.