XC 15-45 or XC 16-50 OR XF 18-55 on X-T20 for beginner ?

Own both XC15-45 and XF18-55, both second hand.

On the wide side, the XC15-45 is by far sharper than the XF18-55 (at least for my copy). Anything wider than 28mm the XC15-45 is much sharper than XF18-55. The aspect that the 15mm gives you much more than the 18mm. Tele side, they perform pretty much the same, both just doing so-so. You need other lens if you want a visually sharp photo on 100%.

Personally prefer the color on XC15-45, but this is just personal.

The problem with XC15-45 is it's electronic zoom. Every time you power on the camera the lens need to zoom external. That's a 1-2 second lag and may cause a few miss-hit if you are using it for street photography or children / pets shots.

The 2/3 stop aperture difference isn't a big deal personally. If you care about boken or brightness it can be a point to consider, while the close-shooting ability of XC15-45 actually generates pretty much the same level of boken.

If you are a photography learner, XF18-55 maybe a better deal. With a zoom ring it helps understanding how you zoom and keep the distance between the camera and the subject.

Overall personally XC15-45 is a very good and far understated lens within X-Series lens series, even only with the wide to mid angle side performance. It eliminated the need for XF16 f2.8 for me, and for it's size you won't bother to bring it with you alongside with other prime lenses.

What prime lenses you will add later on will also be a thing to consider. With XF18-55 at the beginning you will probably skip the XF18 f2, XF27, maybe even the XF/XC35 f2 and XF50 f2, while for XC15-45 you will skip the XF16 f2.8.