The UI kit I use is Balsamiq(https://balsamiq.com/products/mockups/) because unlike Invision's that HAWK linked to, it's clearly cartoony and not the "final design."
Getting the proper user experience correct starts at the element level, and when I've used something so "beautiful" as Invision's, people tend to argue about little design details like color and font, etc. It's not that I don't think font and color are important. But in my opinion, you'll save a ton of time by first nailing down the user flow before getting into the nitty gritty of the final design.
To do all of those things at once can be a big pain in the butt and you'll end up getting the "bikeshedding" conversations way too early in the process. (https://en.wikipedia.org/wiki/Parkin..._of_triviality). When the early UX designs are clearly ugly wireframes, people are less attached and have an easier time looking at the big picture.
My process flow:
1.) Design the user flow with Balsamiq low fidelity mockups.
2.) Work with team to make sure everybody is happy with it.
3.) Send finalized low fidelity mockups to designer to create first version of high fidelity mockups.
4.) Work with designer to iron out any UX issues that arose in the high fidelity designs.