Well, going through all races:
-Orcs: Huntress for quick siege units, but as time passed, I mainly went with Shieldbearers for the frontline, because OMG are they glorious tanks. Sometimes Shamans too. Guess I'll kinda sorta counts the Golems here thanks to Grofzag though, and well, they delivered, and it was nice they could hit air units in a pinch. Definitely not touching the Axethrowers unless they were the only choice though, 420 HP is a bit not enough for me for a 3 Food unit, especially when other 2 Food shooters with roughly same HP can compete in sheer damage and brings gimmicks like Regen, Berserk, Poison... Impalers got them beaten in HP, Damage, and the Crit chance. Pass.
-Trolls: My least used race, generally when flyers were around. Kept forgetting to use them for their Healing Wards didn't help.
-Goblins: Mechas are a solid unit, and Airships were good enough for me against air units (don't recommend them for siege though).
-Bearmen: Gladiators felt a bit redundant with Shieldbearers, although I did use them when I needed to spare some extra food. Elder can help with decreasing enemies' armors, but their healing is my main reason for taking them. Rejuvenation's use makes sure your frontline isn't ever dying, especially when heroes are there.
-Centaurs: I saw the potential, but felt I wasn't good enough on micro for Spiritcallers, and it led me to skip them more than I should have, because tey remain solid units. Impalers were good AA and Missile units, Spiritcallers or not.
-Gnolls: Bringing back Humans. Bloodhounds were nice in a pinch, Assassins excellent, Brawlers... pretty sure I should have used them more to soak Piercing Damage. Wardens (the ones with a Priest's spells list) were a must for me, and Mystics were good when I managed to spare the time to micro them.
-Harpies: Mostly used to repel air raiders and harassment, though I can see the potential for frontline support with Poison, Slowing, and Magic attack can opener.
-Ogres: My sword to the Shieldbearers'... Well, shield. Sometimes with an Overlord for the Aura. Almost always with Magis for the damage boost and somtimes a well-placed Hex. The latter's pretty good melee stats don't hurt either.
-Dragons: Didn't use drakes that much, although I guess it could have been worth a try. Definitely Draongs for snipping critical buildings or siege units while the frontline fought (Easy mode tactics, but they got me through the final mission with Logan alive).
Overall, I went with Shieldbearers (or Gadiators) to tank, Ogres to hit (replaced with high-damage units available when they aren't there), with Magi/Elders/Wardens to buff and heal, Witch Doctors and Shamans when the others aren't here. Shooters were a more... in the-air part of the army, with me experimenting most of them, except Axethrowers, and less and less Trolls I guess. Once I had that core, I went looking for what I'd need considering the opposition in the mission. Frequently no Siege engines though, units can do it too, especially Huntresses, and while the AoE can be good, I generally prefer not having to worry about it, unless they're really necessary. With of course all present Heroes' spells played to maximum effect.
I'd take advices for not too micro-intensive builds to rediscover the campaign though, and builds allowing to progressively learn to use more and more micro-management. Gotta try ot beat these maps on Normal eventually someday...