The dungeons all give the same number of points (assuming you can do stage 20 on all). If you don't care which, then just continue with IG, since super raids will make it faster.

I cant do L20 on all of them.  Dragon is the only one with a 100% success ratio.  I will go back to IG.  

Yea, this Dungeon Divers event is a bit odd in the sense that it only really had Ice Golem running with it.  Champion Training starts on the last day, but it is still very close.  Normally there are 2 other events running parallel so that you can comfortably finish it while finishing other content.

If time is your challenge, then just run Ice Golem using Super Raids.  If you have plenty of time up your sleeve though then run Dragon without Super Raids.  The energy input will be similar regardless, assuming running at similar levels.

If Dragon is the only one with 100% success, then do Dragon. The loss of points per energy from failed runs is just massive. The other option is L16 on IG, but the gear you get from 16 is pretty weak.

I would also do Dragon 20. Ice Golem gear is pretty MEH plus the silver is worse, too. Do the one that's more likely to improve your gear. 

I did IG15 with auto battles and Dragon20 with single battles.  Took a long time.  

I sold about 75% of IG gear.