We only have experience with one tour company. We found them on the internet, and booked them online.
The company is Ukrainian Web.
They seemed a bit pushy when booking (I left my phone number along with my email address when first contacting them, and I got a phone call at 9pm later that night from them), but on the day we had no problems with them!
(We haven't been paid or contacted by this company. We genuinelly did find them on the internet, did a 1 day tour with them and now a few months later I'm writing this. )
Sorry, as we only have experience with one, we don't know about other ones. I don't want to recommend a company here and not know if they are legit or not.