I don't believe in free will. I haven't for some time now. I had to prove it as my final for Philosophy, and I did.
I think the most evidence against free will can be shown by epiphenomenalism. Your brain knows what you'll choose before you do. Based on what's logical (mind-body problem) and psychologically evident, there is no free will.

I found my Philosophy dialogue:
Charles:
As an epiphenomenalist, I believe that mental states, such as happiness, sadness, choosing to move, et cetera, are a result of physical states. As shown by Benjamin Libet, your brain releases chemicals or electric pulses to perform a task milliseconds before you "choose" to perform it. In light of this study, I believe there is no free will.

Ian:
Epiphenomenalism in no way negates free will. Prove me wrong.

Charles:
Epiphenomenalism shows that thoughts come after physical reactions in the brain. Free will is a product of choice, choice is a product of thought, and thought is a byproduct of physical reactions. Therefore free will is a physical reaction and not subject to be changed by any non-physical thing; therefore putting an end to the mind-body problem as well.
This is not all forms of epiphenomenalism, however. Some say that only some mental states follow physical states. I say that's just wishful thinking. Physicalism makes the most sense.

Ian:
But you must remember this very important facet of existence: is this an objective reality we live in? It clearly is not, since we cannot prove anything other than our own mind, as shown by Descarte. The idea that any physical stimulus creates thought is a paradox in itself. How can something not provably real stimulate the only provable process in existence?

Charles:
Sure, if you want to be an idealist and don't believe that physical things exist, but that begs the question of who or what controls your ideals that you do not, such as what we perceive to be reality itself.

Ian:
According to Carl Jung's theory of the Collective Unconscious, it's not "who or what" that controls how we perceive reality. Instead, it's all of us that have ever lived. Because we operate under a collective unconscious, it's all the experiences that everyone has ever had that shapes perception.

Charles:
Yes, but even a collective unconscious cannot, while maintaining that there is free will, explain how your choices can be determined through the physical figment of your imagination before you even choose them. This means some outside force - be it a collective unconscious or a god - knows your choices before you do in order to create that figment. If it isn't feeding those choices to you, it knows what is, and it certainly isn't you.

Ian:
It cannot just be physical occurrences that shape perception. Take, for example, that the majority of people consider hallucinogenic drugs such as LSD to be the most important and life changing experience of their life, and it's just a hallucination; it's all in the head.

Charles:
LSD and other hallucinogens are physical things. They cause physical reactions in the brain, which produce the mental side effects. Hallucinogens do produce a major argument that a reality can "exist" without really existing. However, I see this as more evidence towards physicalism in that the brain is capable of experiencing any reality, but through evolution, only the brains capable of experiencing reality closest to what it actually is survived. People who saw fire as candy or tasted the sweet taste of cyanide didn't make it to spread their genes to what is left of us today.

Ian:
The fact that we are not conscious of it is essentially making for the segue to free will. If there's no way to be conscious of the fact that free will is only technically an illusion thanks to pragmatism, then what's the difference? Yes, everything can be broken down do an action-reaction level, but where's the fun in that?

Charles:
Just because the truth's not fun doesn't mean it's not the truth. One could argue that their false religion is "more fun" in knowing that there is a god who will give them eternal life, but for the sake of advancement of knowledge, we must deny such comfort zones. I don't deny idealism, and believe it holds equal water - especially in the area of solipsism, where your mind is the only thing that exists - but I feel physicalism offers the most sensible explanation based on what we believe we know thus far. It's Occam's razor at work.