1. Sorcerer. Reason should be obvious. Cheapness.
2. Frost Dragons in the ToFR. It's not much fun being repeatedly obliterated within 2 minutes of walking in the door.
3. Anything that poisons you, just because of the annoyance factor of re-ordering your party in the NES version. And the annoyance of running out of antidotes in the Marsh Cave, when you're still poor enough that buying PUREs can be troublesome.
4. Ankylos. If I'm going to kill a walking tank, give me some gold for it.

How can you hate Warmech? He should be admired.

[qq=Sephiroth1999AD]Which monsters cast Confuse? I've beat the game like six times, and I don't think I've ever been hit with Confuse. I only remember using the Wizard's Staff to confuse enemies.[/qq]

Yes, nothing uses CONF in FF1.