If I had to choose just one power? Control/manipulation of toxic substances and disease. Purify food and drink, make alcoholic beverages with some practice, and it has combat applications. Plant manipulation is a close second.
I’d slather on dozens of powers if it was me personally getting isekaied and I had a choice in the matter, but control of the poison “element” is probably what would be most interesting to read/write about in this context.
Unrelated note I think most people overestimate the deadliness of the warhammer setting. Especially if we are talking Warhammer Fantasy. We focus on the worst parts of the world because they are the most interesting, but there are normal people living relatively normal lives all over the place in the empire. Outside of like… the end times, which is apocalyptic, or living in Sylvania, most people can go their whole lives with only the occasional brush with supernatural murderdeathkill, if that.
Warhammer isn’t my first choice, but if I had to choose between Warhammer and Worm, for example? There would be a white board with pros and cons.