Guineapigman, I think it's a tie between chickens and dairy cows.
Chickens endure life crammed into tiny cages with several other chickens, where there is no space for them to take a step in any direction, with cages stacked on top and on the bottom of them, so that they are constantly being urinated and defecated on by other hens. Not too mention, they often attack each other, because they are so stressed and miserable. Seconds after being born, the baby chicks are man handled and thrown around roughly while being sexed. Female chicks get tossed into one pile, and the males the other. And you better believe that no one is going to be gentle in the slightest, as they have 1000's of birds to get through. The females go on to have their beaks burnt off, while the males are either ground up alive (typically for pet food), thrown in a trash can, or other wise deposed of.
A mother cow is often subject to horrendous udder infections, being inseminated via a "rape rack," and typically spends her time standing on hard cement, crammed into a small metal stall, where she is hooked up to machines so that they can take her milk. Of course, that milk was meant for her baby. Like all mammals, the momma cow produces milk for her baby. Yet, for some reason, humans decide that we should drink the milk of another animal species. We "need" the milk, so the cow is inpregenated, and gives birth to a baby. Shortly after birth, the baby is ripped away from her, and chained in a dark crate where they can barely take a step any direction, to become veal. The momma cow will call out for her stolen baby for days afterwards. Technically, by drinking milk, you support the veal industry.
It's hard to say which is worse in my opinion. Either way, male babies are discarded like trash, or tormented so that humans can make more money off of them.
I understand that everyone should have the freedom of choice to live their life how they want it, including diet and lifestyle. However, if the way someone is living their life harms another life, then that's a whole different story.