Octo-ooze: This monster is not meant to destroy towers but to stall them. That is its only purpose. I would suggest only using this until you unlock the Ichi; then abandon it.
Ichi: One of the most powerful monsters in the game (with Zafreeti, Teratorn, and Project X) and my personal favorite monster, Ichi is quite good. It can soak up damage and in large amounts, one-shot just about any defense tower. I would suggest distracting with this before flinging in Project X as your strategy(with Zafreeti if you can) as it can destroy every base I have come up against.
Crabatron: NEVER USE THIS MONSTER! It doesn't have enough damage to be a threat, and 2 Ichi of the same level have higher health,speed and damage. Also the Ichi goo cost is far less. Considering it is harder than Ichi to unlock, unless they nerf Ichi (don't do it KIXEYE, please!) I STRONGLY recommend against this monster. Even with splash damage, it's not worth it as they end up with about 1,000 less health than 2 Ichi. This is enough health that Crabatron can only really be used to scout for traps after all the towers are gone (but Bolts and Pokeys are better at this and less expensive)
Project X: Unlike the Crabatron, this is quite a good monster. while it has low health, it's high damage can be used with Ichi to obliterate most bases, if not all bases. Ichi(or maybe Crabatron but I wouldn't suggest that) distacting towers with Project X destorys them is my favored tactic for attack.
NOTE: I didn't include Inferno monsters, but I will when I finish unlocking all of them.