For lazy player like me, usually they are:
1. Those spawn spikes of fire in sewer act 2 (need full fire resist)
2. Daystar (same reason)
2.1. Shredder and Black Cat in arcane sanctuary
3. Ice clan (need full cold resist)
4. Force-waller in act 3 jungle
5. Combination of mobs in lower kurast (pain in the ass if you surrounded before finding the WP)
6. Elite mobs that fire homing skulls in act 4
7. Groups that cast coldstorm (cold version of bloodstorm)
8. Detonator
9. Stone eyed gargoyle? (that cast SoR)
10. Thundershaman + Minions
Probably more, but most of those are mobs that prevent me from rushing through level without fully upgraded gear.