Best Applications for UI?

I started out with Paint .Net - a really powerful tool for creating nice photos - however with UI you want something like @Kamfkarren says.

Inkscape is good and free - you can convert raster (.jpg, .png - normal images) to vectors which is handy, however dealing with colours is a bit annoying.

I recently purchased Affinity’s line of photo/vector editing software - Affinity Photo and Affinity Designer. They’re both excellent options and are one-off (and very cheap) payment.

Photo handles raster images and can even process RAW footage.
Designer is all about Vectors.

If you have a graphics table you can hook it up to both tools as well.

I’d suggest starting with Inkscape first, and then compare Adobe Illustrator and Affinity Designer if you want to get serious.