smoke mephits, definitely smoke mephits.
**Slight Spoilers**
Golems were really tough until I acquired the right weapons. Beholders and Gauths were really tough until I acquired the cloak of mirroring. Dragons were extremely tough until I found out that Mordakian Swords are immune to all of their damage. In the end I'd have to agree with you MindFlayers are the worst; at least when they attack in packs.