>>725164205 (OP)
Some units you need to upgrade instantly:
[Castle]
Marksmen - double shot
Crusaders - double strike
Champions - one of the best 6th tier units
Angels - one of the best 7th tier units
[Rampart]
Centaur Captains - speedy as fuck, great for finishing shots
Grand Elves - double shot, one of the best archers
Silver Pegasi* - only if you don't use autoresolve
Gold Dragons - dragons are very good
[Tower]
Master Gremlins - ranged attack
Master Genies - even one can buff your power stack
Naga Queens - so they can at least reach enemies, plus one of the best 6th level units
Titans - ranged attack
[Inferno]
Magogs* - only if you use HotA
Cerberi - no retaliation and 3-hex attack
Pit Lords - demon farming
Archdevils - they're just good and even better with HotA's luck rework
[Necropolis]
Vampire Lords - win you the game
Dread Knights - one of the best 6th level units
[Dungeon]
Harpy Hags - no retaliation
Medusa Queens* - or have an ammo cart
Minotaur Kings - they actually get one of the most significant stat buffs out of all creatures in the game
Black Dragons - magic immunity
[Stronghold]
Hobgoblins* - you'll need to upgrade them for Wolf Raiders anyway
Wolf Raiders - double strike
Thunderbirds - lightning strike is a significant damage boost early-game
Ancient Behemoths - can get them first week in ideal circumstances
[Fortress]
Dragon Flies* - if you don't use autoresolve
Mighty Gorgons - death stare
Wyvern Monarchs* - if you can do it very early (like turn 4)
[Conflux]
Storm/Ice Elementals - ranged attack
Energy Elementals - flying
Phoenixes* - their growth is broken if you're not using HotA
[Cove]
Sea Dogs - effect similar to death stare
Ayssids - situational double attack
Nix Warriors - very tanky
[Factory]
Halfling Grenadiers - good against high-tier units early
Sentinel Automatons - no retaliation
Bounty Hunters* - only if you're going against enemy player's ranged stacks
Crimson Couatls - 1-turn invulnerability