Depends what you want to clean off.

If rust, Evaporust is super. Saves a ton of work and makes rust disappear just by soaking the part. Remember it is reusable. Don't forget rust converter for parts you can't use Evaporust on. This will turn the rust into a stable oxidation.

If grease, then I use a citrus degreaser and a toothbrush.

But often, it is easiest to just replace the bolt. I generally try to replace stock Grade 8 bolts with stainless bolts. However, if the bolt interfaces with aluminum, you need to make sure to grease it really well, or you risk corrosion.

In the end, cleaning a rusty and grungy bolt is a labor of love and probably only worth it in circumstances where you want to preserve all the original parts, a vintage QR skewer, for instance.