Definitely JPG, as it’s the most ubiquitous file format out there that everyone can view without needing special software.
Recently I’ve been delivering files at 3300px long on the long side, which I think is a good balance between high resolution and smaller file sizes. (3300px is big enough to print at 11 inches long, a standard paper size.)
My sister has a website the I shoot the pictures for. I give her a high quality JPA and then a “web-ready” smaller file with all the size requirements for her site. The “web-ready” I label as such in the file name. Both of those I have set up as an export preset in Lightroom.
It works well for us. If she wants to crop or print she just uses the “original”.