Here is my tier list:
Explanation:
Skele and Witches are a brutal combo and need no explanation, Witches would be C tier on there own, but I never send them alone as this is ranked off my use of them and my tactics, Spiders, and Pigmen are A because they can be lethal to a slow opposing team if your quick and smart enough, I have one with this grouping more times than any due to the unpredictable factor, Slimes and Blazes are also good but the issue is cost, I prefer cheaper and quicker, I still use them late game sometimes, Creeper and Silverfish are just average, not bad, not good, never use them, but still noteworthy, Finally, At D we have Zombies, clearly they are only worth quick XP grind and then forgotten about forever, Giants are not worth it, there only semi-good when they are levels up to max, but you can do more damage quicker with anything else almost.
Open to debate and discussion as I love this game!