I liked the Sylvans.
Here's a description of the alternate upgrades:
1) Dryads - Nymphs. Nymphs lose the ability to cast spells but gain the ability to heal Ents, Ancient Ents, and Wild Ents, and receive healing from them in return.
2) Dancers of Death - Dancers of the Wind. The new dancers lose the ability to attack adjacent creatures but increase their defense against counterattacks for each cell they move before attacking a creature.
3) Elf Archers - Forest Archers. The new archers have no penalty for shooting and have higher attack power. They lose the ability to paralyze enemies and double shot.
4) Elder Druids - Ancient Druids. The new druids forget the Lightning and Mana Transfer spells but can increase the hero's spell power (depending on the number of druids).
5) War Unicorns - Light Unicorns. The new unicorns lose the aura that protects against magic. However, the "Child of Light" ability allows unicorns to absorb any light magic (except Divine Retribution) cast in battle by any creature or hero.
6) Ancient Ents - Wild Ents. The new ents can no longer hide their roots underground. However, they know the "Fury of the Forest" ability, which reduces their defense by 2 times and increases their attack by the amount of defense lost.
7) Emerald Dragons - Crystal Dragons. The new dragons lose protection against earth elemental magic but unleash Rainbow Flame, which has a random range (1-5 cells), depending on luck.

From the first option, I always choose nymphs because dryads have weak magic.
I choose dancers depending on the number of enemy units.
I always choose the alternate archer upgrades until I get the Unicorn Bow artifact (removes shooting penalties) because it's better to deal double damage to the enemy without a penalty.
I choose druids depending on the hero's magic (if the hero knows weak magic or doesn't know any magic at all, I choose the regular upgrade).
I choose unicorns depending on my hero's knowledge of light magic and the enemy's magic.
I choose the regular ent upgrade until I learn spells that increase defense.
I also choose dragons depending on whether the enemy knows earth magic or the number of units in the enemy army.