Only a few silent films have been given the level of digital cleaning that Metropolis has gotten. It is simply flawless. Pretty much all other silent films you come across are going to show at least some scratches and flaws. That being said... There is still plenty of variability in the quality of transfers.
The Kino version of "The Cabinet of Dr. Caligari" has some rather serious problems in the opening scene, but after that I'd say it's superior to the Image version in pretty much all aspects. It's pretty good I'd say.
Speaking of "Buster Keaton Films"... There is a recent Image double feature with both "The General" and "Steamboat Bill, Jr." While it's a good deal, if you have a progressive scan player or use a HTPC you will probably notice shimmering and other interlacing artifacts. If you can afford it I'd recommend the Buster Keaton box set from Kino instead. It has better transfers of both of these films as well as many others. I'd have to give the edge to the soundtrack's on the Image double-feature though.
The recent R1 Charlie Chaplin Box set from Warner Brothers has a transfer that shimmers absolutely horrendously, but is otherwise quite good. I've heard the R2 version might not have the same problem, but can't confirm that. Here's hoping the second box set they're coming out with shortly is improved. Chaplin's films, "Modern Times" in particular, are simply enchanting however and I strongly suggest you find a way to see them. ("Modern Times" actually has a soundtrack, but it is still done silent style.)
If you like documentaries, I suggest you check out some of Joe Flaherty's work. Criterion has put out a serviceable transfer of "Nanook of the North" and Image has released an excellent DVD of the stunningly gorgeous "Tabu". ("Tabu" is actually not a documentary, but a collaboration between Flaherty and the Murnau, of "Nosferatu" fame) There are also excellent DVD's out there of "Man of Aran" and "Louisiana Story" that are both stunningly beautiful, although neither is silent.
Speaking of F.W. Murnau, his most famous film, "Nosferatu", was given an excellent transfer and accompanied by quite a good score by Image. (There are a tonne of different versions of Nosferatu on DVD, so be particularly choosy here) Kino has put out a good transfer of "Faust", which is an utterly amazing film to watch. Many of the special effects Murnau used look better than some you see today! Murnau's masterpiece, and perhaps the greatest silent film ever made, "Sunrise", is available from Twentieth Century Fox, but only as part of a box set or through a mail-in offer. This is another title which I strongly recommend you find a way to see. It is simply superb.