Here are highly regarded guided‑tour companies, grouped by travel style so you can pick the type that fits you best. I’ve included one short note about what each is known for and a source you can check.

Small‑group / responsible adventure

• Intrepid Travel — small‑group adventure operator known for strong sustainability credentials and B‑Corp certification. (intrepidtravel.com)
• G Adventures — budget‑to‑midrange small‑group adventure tours with a strong focus on community projects and experiential travel. (gadventures.com, en.wikipedia.org)

Expert‑led city & walking tours

• Context Travel — expert‑led, small private walking and museum tours (scholars, curators, chefs) in 50+ cities. (contexttravel.com)
• Rick Steves’ Europe — U.S.‑focused, Europe‑only small‑group itineraries geared to American travelers who want cultural, well‑paced tours. (ricksteves.com)

Marketplaces / last‑minute & single‑day activities

• Viator (Tripadvisor) — huge searchable catalogue of bookable tours, excursions and shore‑excursions worldwide. Good for one‑off activities. (tripadvisor.mediaroom.com)
• GetYourGuide — large global marketplace for activities, tickets and guided experiences with many instant‑book options. (en.wikipedia.org)

Luxury, tailor‑made and high‑service operators

• Abercrombie & Kent — long‑established luxury operator (safaris, private journeys, expedition cruising) with boutique service. (abercrombiekent.com)
• Tauck — premium escorted tours, river cruises and family/heritage programs with decades of experience. (tauck.com)

Expedition & wildlife specialists

• Lindblad Expeditions / National Geographic — top choice for small‑ship expedition cruises and expert‑led wildlife/nature trips. (expeditions.com, world.expeditions.com)

Private local guides & fully customizable local tours

• ToursByLocals — marketplace that connects you to vetted local private guides for fully customizable private tours worldwide. (toursbylocals.com, en.wikipedia.org)

Quick tips for choosing the right operator

• Decide style first (small‑group, private guide, luxury, expedition, day tours) and compare group size, itinerary detail, inclusions and cancellation policy.
• If sustainability or community impact matters to you, look for B‑Corp, GSTC or clear responsible‑tourism policies and ask what local partners/charities they support. (intrepidtravel.com, lonelyplanet.com)
• Read recent traveler reviews (Tripadvisor/Trustpilot) and check the operator’s on‑the‑ground support/contact policy.

If you want, I can narrow this list to the best options for a specific destination, travel dates, budget or travel style (luxury, family, active, food‑focused, safari, river cruise, Antarctica, etc.) — tell me the destination and your priorities and I’ll give a short, tailored shortlist with links and booking tips.