Go to TripAdvisor.com and check out their reviews of various companies, tours, guides, museums, etc. The details there aren’t always up-to-date, and they’re often biased unfairly in a negative direction, but I think this website is about the best source of such info — at least if you can take the time to read with a critical eye.
Anyway, for me, a guided tour of several days thru an entire area is usually best, because I can carry on a extended dialogue about local history and geography with the tour director while we’re on the bus, at restaurants and elsewhere. I learn a lot that way. That was certainly my experience when touring Normandy.
Moreover, the typical guides and tour directors for just about any tour company will know which times are best to visit certain sites, so as to avoid crowds and early closing times. And they will know which sites are worth skipping, especially some of the tourist traps. Also, other tourists in your group often will have in-depth knowledge of particular topics that can be valuable.
My wife and I have used three tour companies recently in Europe: Grand European Tours, Road Scholar and Grand Circle Tours. All were very good, with excellent tour directors. And I’ve heard good things from friends about several of the other big USA and British tour companies.
But the guided tour is not for everybody. Maybe you will enjoy the sites more if you simply hire a local guide at certain sites, then travel on your own from place to place. That approach is usually not my cup of tea, but then I wouldn’t want to judge your personal preferences on the matter.
Further removed from my preferences, some people I know evidently get a certain “satisfaction” from figuring out EVERYTHING by themselves when they travel overseas. No guides anywhere, period. I think they’re seriously short-sighted in this regard — but to each his own.