I have traveled with the following tour companies -
Adventures by Disney - Italy Signature (this specific tour is no longer offered), Alaska, San Francisco (long weekend), and Montana/Canada
CIE - Ireland, Northern Ireland, Scotland, England and Wales
Tauck - Germany, Austria, Switzerland, Lichtenstein, and northern Italy
G Adventures/National Geographic - Costa Rica, Morocco, and Ecudador
Rick Steves - Sicily
I would do ANY tour by G Adventures/National Geographic in a heartbeat.
I'd do specific Adventures by Disney again. (Long story, so maybe not relevant here.)
I got my money's worth out of CIE. (GREAT tour companions.)
I hated Tauck, despite the fact that it's the 'high end' tour company.
Rick Steves? We had an awful guide. The tour was amazing. The guide was not. Apparently, this is not the norm.