Try to look at this:
Generally speaking - more to the right side of the chart are potentially more powerful items. This chart is not perfect but I’ve created it to visualize itemization in Grim Dawn Better. I hope it will help you a bit with understanding the itemisation
Still you can get double rare that is utter crap for any build… unfortunately ;( Need to praise to RNJesus And how to know what is utter crap? Well that comes with exp. Same as in PoE. You can have spell dagger, phys dagger. Same for 2handers, etc. etc.
One of the general rules is: I’ll say that +1 to any skills are almost always better than +%dmg.
But still not true for every item. blah blah blah…
Just play and learn