[Verjigorm]

Level 1: Black Servant
Level 3: Sleeper
Level 4:
Level 5:
Level 6: Spectre, Most Ice Devils
Level 7: Wraith Lord (Immortal!), Ivy King, Some Arch Devils Father Illearth, Vampire Lord (Immortal!)
Level 8: Lichcraft (Immortal!), Some Heliophagi

The End-game Diversity mod expands not only late game summons bud mid-game thug summons as well. Check it out. It's becoming increasingly popular and as it gets more wear on it, more well-balanced.

If you're looking in blood, the best SC/Thugs IMO are the Ice devils since all of them come with greater than 20 basic protection and Cold Power which means you can slap a Robe of Shadows on them. They can also cast Quickness on themselves, so you can substitute Boots of the Messenger for Boots of Quickness giving you versatility in your miscellaneous slot. The more melee focused ones also have good weapon alternatives--one has a Hell Sword (Life Drain) and one has a Frost Brand. Nycafor has pokey hand thingies that you can't replace, but he's still decent. They are also terrifying underwater opponents especially considering how squishy most underwater stuff is and how fast/cheap you can get them. The Arch-devils are more well-suited to land adventures and are not generally as powerful. I tend to use the Heliophagi and Arch-devils as artillery and the Demon lords cannot use all the equipment slots so they tend to be used either for their special ability (Belphegor/Pazuzu), casters, or the dudes I use to cast "army" spells.