aside from some modifiers that grant awesome skills, like ex Outcast Hood or Apocalypse, and other “flashy skills”
you have stuff like that makes a 2h exclusive skill able to be used with 1h
making certain cooldown skills have less cooldown so you can use them more often, almost "spam"able, ex Vanquisher set for skating, or Loxmere blades for Shadow Strike
obviously dmg modifiers that transmute skill dmg type to another so you can use the skill in a different build, ex Fire Forcewave and such (there are some really nice FX changing transmuters, like green Sigils etc)
Passthrough is super cool, tho rare, it allows you to “shoot through enemies” with a skill/shot, so all get hit, and this can look pretty awesome with certain skills, ex @eardianm’s famous dudu
stuff that adds “aoe”/multi target hit to skills, so when you swing your big club you hit like 5-8 targets instead of 1
would be impossible to list them all i think, there are so so many, and they don’t even all fit on 1 build but you have to do many different characters to “try them all” 
but overall i think stuff that either changes the FX or effect of a skill is my favourite modifiers, because not only do they make things potentially look cool, they also add so much different utility to the items/skills so you can use the same skill with a different dmg type or get “more” out of a simple skill, ex more pet limits so you get 2 dogs, or 4 blade spirits/guardians etc etc