It depends on what the person values. There is no one size fits all argument.
Ethical (onion)
- First Layer, abuse.
- Second, suffering.
- Third, killing.
- Fourth, exploitation.
To directly answer the question, talking about a case of animal abuse and segwaying into inflicting unnecessary pain/suffering (namely factory farming) has the most clout for most people (IMO).
Environmental
Pulls a lot of environmentally conscious people.
Health
A few.
Here areis a bunch of related studies: