Along with K&P, makes for two of the best, and where Kernighan was a co-author, IMHO.
Of course, caveats apply and should be mentioned (a few others have said it), such as it not being up to date with latest versions of the C standard. But that does not diminish the achievements or quality of K&R.
However I don't think it is necessarily the best first book on C for everyone, but it can be for some. Depends on the amount of systems-level and close-to-the metal knowledge they have; at least a minimal understanding of how computers work under the hood (RAM, hard disk, CPU, files, etc. [1]), or at least a mental model of that, helps and those who have that will do better with K&R as first book. Others may be better served by an easier book and then should move on to K&R.
[1] And some people, even on here, may be surprised at how much some people are lacking in that sort of knowledge, more so in these days of Android and Apple tablets and smartphones, where a lot of the underlying details of computers (both on hardware and software) are abstracted away. Very different from the age of hobbyist home computers, where you had to know a good amount about the internals to use the things, as some many comments in recent threads here about them have mentioned.
