Material Bootstrap is by far my favorite UI system because it takes the core of bootstrap, its smart grid, and combines the Material design of Google to make a simple, out of box UI that creates sites that look professional and unique with minimal customization. They have a free plan and a PRO plan that includes templates and exclusive sections that I think are totally worth the price. Its fluid animation, design, and add-ons make this comprehensive UI system by far the best. Its also super helpful that they have taken a lot of time to perfect their CSS and JS components that go with their building blocks to give you some easy animation. The best part is that it is still lightweight and the material design makes it sleek and welcoming on all screens.
PROS: Almost unlimited components and layouts, professional design, unique animations that can be added anywhere
CONS: Has paid components, code can be bulky... Seriously, thats it.