I play as Sylvan and I chose:
Level 1 - Nymphs. I think they are more useful than Dryads. After all, they heal Ents!
Level 2 - Dancers with the Wind, I think they are better than Dancers with Death. Because a direct, strong attack is better than an average attack on everyone standing around. Especially for level 2.
Level 3 - Master Archer and Sniper, 50% each. There are different opinions here. What is better, one super-strong shot or two strong shots?
Level 4, I haven't figured out who is better among the Druids yet.
Level 5 - Battle Unicorn, I think it's better than the Light Unicorn. It's better to have magic resistance.
Level 6 - Wild Ents are better.
Level 7 - Crystal Dragons are better, I think!:)

Choose the new Druids, as it's better to give the spell power to the hero. For example, I had 100 new Druids and created a channel. Now I revive more than 10 dragons with Resurrection. But if you don't know magic or know bad magic, it's better to use the old upgrade.
Regarding archers, choose the Forest Archers because one super-strong shot is 100%, but if you have the Unicorn Bow (which removes the penalty for shooting), then the Master Archer (who shoots 2 shots with 50% damage each) will be better (2 super-strong shots are still better than one).
Regarding dragons, yes, Crystal Dragons are better, but think about it, what if the enemy knows the Earth Shock spell (strong earth damage)?
Conclusion: each upgrade is not better than the others; they just have different abilities.