I'm sure there are many good companies so I can't say which is "best", but I used ClimbKili in August 2020 for the Machame route and they were very good. Price, ethics, organisation, communication, Covid-policies and flexibility in dealing with flight changes due to Covid, were all excellent. The guide and crew of porters were really great, as was food, tent etc. One nice bonus with climbkili on the Machame route is that they use Camp Kosovo as the high camp instead of Barafu. This shortens your summit morning by an hour or so.