Some of the most popular genocides are:
- Liches (due to powerful magic like destroy armor and touch of death)
- Mind Flayers (due to their brain-eating attacks)
- Sea Monsters (due to their ability to drown - a 1-turn kill)
- Rust Monsters & Disenchanters (due to them weakening weapons and armor)
When you have a scroll of genocide, you should consider waiting to bless it before you use it. A blessed scroll wipes out an entire class of monster, while an uncursed one only wipes out a single species.
You may also consider cursing your scroll of genocide. This will create 4-6 monsters of the chosen species (commonly referred to as reverse-genociding). This can be useful. For example, some people prefer to reverse genocide silver or grey dragons to get scales for DSM, rather than having to use a wish. Others pick wraiths to eat for levels, or giants to eat for strength.