This is a loaded question. I would hazard a guess and say protein, because gear is going to allow you to do more with protein, but if you don't have enough, it can only do so much.
After that, it would depend on if you have any orals. If so, I would add some n-acetyl cysteine, and/or calcium d-glucarate. I have no idea why people think milk thistle is that effective, compared to those two I just mentioned, taking milk thistle is just pissing your money away.
Obviously, you want to make sure you're getting all your nutrients, you don't want to have some kind of deficiency that takes away from the effectiveness of your gear.
And, not that it's a supplement you buy, but getting enough sleep is huge.
That's about as much as I think can be answered on a question that is pretty much difficult to answer.
Protein, micronutrients, liver protectors, sleep.