Haven

Brute (or unupgraded in castle for extra money)
Crossbowman (unless Unicorn Horn Bow)
Squire
Battle Griffin
Zealot
Champion
Archangel (unless defending a castle or have Master of Blessings)

Sylvan

Sprites (split into stacks of 1) while creeping, Dryad when you have a full army
Wind Dancer, but don't waste money on them
Arcane Archer (unless Unicorn Horn Bow)
Druid Elder early for Lightning Bolt, High Druid once you have a big stack
Silver Unicorn (unless skill in Light Magic or vs Haven/Sylvan)
Savage Treant?
Crystal Dragon (unless vs Inferno/Dungeon/Academy/Fortress)

Necropolis

Skeleton Archer (could reupgrade after creeping)
Rot Zombie (unless vs Necropolis)
Poltergeist, but don't waste money by buying more than a couple to split up
Vampire Lord (unless you're a big gambler)
Lich Master
Banshee
Spectral Dragon

Inferno

Familiars
Horned Grunt
Firehound
Succubus Seducer (unless vs Necropolis)
Nightmare (unless vs Necropolis)
Pit Spawn (unless you're rushing to the enemy castle right after getting the upgrade)
Arch Devil (but it's close, especially very late game)

Dungeon

Stalker while early creeping, Assassin when you have some army
Blood Fury
Minotaur Guard
Dark Raider
Foul Hydra
Shadow Matriarch
Black Dragon (unless vs Sylvan/Stronghold)

Academy

Gremlin Master while early creeping, Gremlin Saboteur when you have a full army
Obsidian Gargoyle (unless skill in Destructive Magic AND you're NOT vs Inferno/Dungeon/Academy/Fortress)
Magnetic Golem (unless NO skill in Destructive magic AND you're NOT vs Inferno/Dungeon/Academy/Fortress)
Archmage (unless skill in Tactics)
Djinn Sultan (unless vs Inferno/Dungeon/Academy/Fortress)
Rakshasa Kshatra
Titan?

Fortress

Shieldguard
Skirmisher
Blackbear Rider
Berserker?
Rune Patriarch
Thunder Thane
Magma Dragon
Stronghold

Goblin Trapper (could reupgrade after creeping)
Centaur Nomad?
Mauler
Sky Daughter (but it's close)
Executioner
Paokai
Untamed Cyclops