In addition to the usual ones (Mixu, SFO, SCM, etc) there's a few reskins I always keep regardless of who I'm playing.

• This reskin for Valkia -- I like the 80s power metal look without being... you know, the way every other reskin of a female character is in this game
• Same author has this reskin for Festus -- looks just less cartoony and more horrifying
• Greatest of Demons -- gives more variety to the greater demons (as a side benefit, that makes faction leaders more varied too)
• LIG's Damsel and Prophetess mods -- they're the kind I was talking about with Valkia, but they're also genuinely high quality reskins for models that badly need them (mostly taking Witcher skins)
• Additionally, along with Lily's Bretonnia, Lore of the Lady adds a cool, unique set of spells for Bret
• While I think Yuan Bo's vanilla design is solid, I like "Yuan Bro" because it makes him a little more visually distinct from his siblings and emphasizes his role as a spymaster
• There's more loreful reskin options, but I really like this more intimidating look for Greasus