Atmosfair (https://www.atmosfair.de/en/offset/flight/) has received positive press and reviews (https://www.test.de/CO2-Kompensation-Diese-Anbieter-tun-am-m...) in Germany.

Its calculation also includes the flight class, aircraft type, flight type (charter, scheduled) and they documented it as well (https://www.atmosfair.de/en/standards/emissions_calculation/...). We use it to offset our business flights. It's a charity and donations are tax deductible under German law.