Guided tours have basically two different types. The regular ones that highlight the collection of the museums which may be offered on a regular hourly basis. The tours with special topics, on the other hand, may either link different objects that people usually do not associate together or interpret some collections from a unique perspective. For the obvious reasons, the special tours are not offered frequently, at least not repetitively.
But there are actually not too much difference between the two types. To some extent, both are unique. Unlike the museum labels or audio recordings, human beings are subject to change. Not only that each individual guide is different in personal preferences, backgrounds and presentation tyles; but also each tour may differ to tailor the specific interests of the presenting visitors.
Most museums for sure give the first type of guided tour. But it is the second type that I am particularly fond of. The general guided tours tend to focus on the objects themselves. There are pieces of facts here and there: who, when, what content, what material, what technology, what did critics say and how it is praised now etc, the same of which one may be able to find through some online search; but there is seldom connection in between.
However, special topic tours narrow down the selection that would make a presentation more meaningful and coherent. Frick Art Center in Pittsburgh does not have a large permanent collection, but its seasonal special exhibitions ("William Bouguereau and his pupils,", "From J.P. Morgan to Henry Frick", etc) were always intriguing. For such special exhibitions, curatorial efforts are more prominent because the show (how the works are presented, what works are selected) is as much based on the artistic value appreciation as a show case of either a scholarly or museological study. To look at individual ones may miss the whole point of the curatorial perspective, which makes each such exhibition special. (In contrast, artworks in the permanent collections are there on the wall possibly because some rich guy donated them 80 years ago.)
Thus, large museums, because of their colossal collection breadth and depth, are in great advantage to give such special guided tours. Met has a room dedicated to Corot and has more than multiple paintings of Caspar David Friedrich accompanied by other painters from the golden age of Denmark. Under such circumstances, there is a chance to learn systematically and methodologically through examination between time-line, location changes and through pair-comparison.
I remember the fascinating interview of an avid collectior in the "Art and Antique" magazine. Collecting evolves through different phases. There is a time when you spend 5000 dollars to buy a painting and immediately hang it on the wall with pride. Then next you may find you are struggling to find a suitable spot for a 50,000 dollars painting. Ultimately in the last phase, you may immediately put a painting worth half a million into your storage place. I may sympathize the "poor" rich collectors trapped by the limited space, but on the other hand what are shown in his living space must be well wrought through his accumulated years' experience and knowledge; thus it did not surprise me collectors at this level featured in the magazines display their collection in an organic and breathable way, almost yearning the visitors to listen to their stories.
Museums can be regarded as the ultimate collectors that have put in storage artworks that otherwise would be worshiped like shrines at individual's home. But that does not make the curatorial effort of presentation easier. Brooklyn Museum of Art displays artworks based on topics: landscape, genre, portrait, religion, etc; while Carnegie Museum of Art divides art history into several periods (in general a few decades) and displays artworks based on their creation date. Probably only Met has the luxury to display artworks by each school or even each individual. Under whatever way or order artworks are organized, they are missing other linkages or presentation possibilities.
Even at the Met, when one is surrounded by Corot's poetic landscape, he or she may fail to see how later impressionists were influenced from his plein-air execution since these galleries are not directly connected. On the other hand, there are some story telling display within the museum such as period rooms that the collection of objects works together for a vivid narration. More or less, it is up to the curator to fabricate a convincing period settings with some original, some fake objects to convey a sense of history, artistic and social context. There are so many nuances such as the position and size of windows, the fabric on the tables or the cushions of some chairs that without being told visitors can hardly grasp the purpose of the exhibition. In both cases, special topic tours would be beneficial or even important for the enhancement of museum experience.
Here are the links to the calendar of the major museums.