I take you mean which monsters are better chosen than others, not which monsters can be used in which scenario (there is a PDF listing all the monsters available for each Encounter). If you want to be mean, the choices start with: Can I use the Giant? If yes, then Giant. Assuming you have the CK. Heroes don't have area-effect attacks? Kobolds, kobolds, kobolds. Crazy with Unholy Ritual to boot, even worse with Reinforce thrown in the mix. Act II master Shadow Dragon with Frenzy + Bloodrage = potential 4 attacks with Fire Breath, each capable of doing 8-10 dmg to ALL heroes if they stick together. Until the heroes get reliable skills/equipment for surge creation outside what the dice show, Shadow Dragons are very good even so.