Royal Snake is definitely the best unit in the game
and they are even more deadly when you have the frog wife. If you are lucky enough to get some fire spiders early on, they and the royal snakes will stomp anything standing in front of them
(especially with the Jade belt which gives bonus for spider). RS is a must have for me till I find other good combo in late game. One more unit that is vital in my army is Evil Beholder, its mind control and sleep ability is fantastic for crowd control.
Strangely enough in the snake and spider family, only those 2 I list are excellent , the rest of them are either just too slow (snakes) or too weak (spiders) and not worth using. I also dislike the Ent (high lv elf unit) because of their -100% to fire, slow and limited range ability. The rest of the units have their uses, you just gotta find the right way (and equipments) to maximize their potential.
Black dragon is the best of all, I dont use the others often because when they fight other units that has much more LDR they suffer heavy casualties and become weaken fast.