Like in any other strategy game, some creatures are well-suited to countering certain other creature types in MW:
For example:
Iron Golem counters anything that inflicts conditions it is immune to; ie burn/rot/weak/push.
Lord of Fire counters bird swarms, because it is flying and sweeping.
Darkfenne Hydra counters any non flying melee swarms.
Feral Bobcats are highly effective against single large units (except hydra), because they are cheap and have a Defense roll.
Creatures with Defense rolls counter Vampiress, since she loses an entire round of healing when her attack misses.
Creature swarms are, in general, strong against creatures that have a single Defense roll (ie Knight of Westlock)
Burn tokens are, in general, strong against high-Armor creatures that aren't resistant or immune. (ie, Knight of Westlock, Grizzly Bear)
Daze/Stun tokens are, in general, extremely strong against anything level-4 or higher, average against level-3 and weak against level-1 or 2.
Anyone have other things to add to the list?