I love prime lenses for everyday use and would also recommend one based in what you’ve already got. Your two options would be either 35mm or 50mm, pretty standard lengths so lots of choice. The 35mm will give you the equivalent of 50mm on your sensor and would be the better choice if you wanted to use it primarily for street photography and general snaps while out and about. The 50mm would be the better choice if you were going to use it primarily for portraits. Portraits work better with a longer lens as the foreshortening effect of the focal length compresses facial features and results in a more flattering presentation.
I’ve had in the last a Nikon D200 with 50mm lens that opened all the way up to f1.4. It was brilliant for portraits but while f1.4 really gives you hand holding options you wouldn’t otherwise have, you do have to watch the depth of field. You can end up focusing on the eyes and still have the nose and other features blurred.
F1,8 would be cheaper and only marginally less flexible and even then only really valid on the 50mm for portraits. I going for a 35mm, I’d suggest you could easily work with something like f2.8 or maybe f2.
Sigma lenses tend to be a lot cheaper than OEM lenses as well.