I think all these creatures can be useful in their own right.
Conscript/Brute, I don't know about, because I'd rather never upgrade that dwelling until the very end, making money off the peasants. But I suppose the unupgraded creature can be factored in as well.
Familiar/Vermin, well I think the Familiar are probably the worst creatures in the list, but the Vermins are actually quite useful to give your Pit Lords some extra mana, which is good especially in combination with a hero that specialises alot in Gating.
Skeleton Archers aren't very good, but you usually get alot of them, and beside the Liches (and if you choose the alternative upgrade for Liches, you'll be using them alot for spellcasting anyway) are the only shooters in the Undead army. Still, I think the Skeleton Warriors are better. They can become the strongest stack in your army..
Sprite/Dryad.. Well, I never much liked the Dryads, as they are usually dead before they can use their ability to heal the Treants, but Sprites are really good. I had a 3x3 duel where at the end of the battle the Guardian Angel ability kicked in and resurrected more than a hundred Sprites from the first hero. Since the enemy hero was out of mana (Sprites don't stand a chance against destructive magic..), I was actually able to win because they managed to outrun the enemy troops enough for my hero to finish them off.
Assassin/Stalker: I always liked the Assassins, with their poison. But I gotta say, nothing beats Stalkers.. For reasons already described
Master Gremlin/Gremlin Saboteur: eh.. Master Gremlins are kinda bad. Even a large group of them doesn't do much damage, and resurrects way too few golems. Maybe they're useful if your first aid tent is destroyed..
Gremlin Saboteurs don't have much use either. That is, until you meet an enemy Sylvan hero with Triple Ballista, Imbue Ballista and Fire Arrows. That's when these guys are incredible
![smile_teeth :D](/forums/images/smilies/smile_teeth.gif)
Still, they won't survive long enough to sabotage the ballista once, maybe twice.
Shieldguard/Mountainguard: maybe Stalkers have invisibility to keep your hero casting spells 3 more turns, but if you play it right these guys can keep you hanging in there for quite a while as well. Plus it's just great to have a bunch of them in town defense
Goblin Trapper/Goblin Witch Doctor: these are both the best and the worst level 1 unit. Worst, because, well.. I've never actually noticed Defile Magic to work, and those traps maybe once or twice. They can still give a nice hit once you've progressed into melee combat, due to their numbers, but that's it really. Best, because you need them for the Shamans and Cyclopses.
I rather compare in player versus player fights, which means Stalkers aren't going to get my vote
![smile :)](/forums/images/smilies/smile9.gif)
I think I'm gonna go with Vermin. Giving your Pit Lords more mana is really a good ability.