Nope. Not my favourite, and not by a long, long shot.
This possibly amounts to sacrilege for may hardened fans (and there are lots of them), but seriously, no, I quite disliked the book. This was my first Stephen King and the netizens/citizens surrounding me have created a pedestal for King, which, though having never read one myself, I was apt to accept and respect. So when I found that the novel was littered with useless slangs and really going nowhere unpredictable with the plot, I was crushed.
As rightly guessed, the language was my biggest concern. Call me old-fashioned or what, it seemed too juvenile to be using such parlance any more; it was too flippant for my taste, and in some parts, outright cheap (though in some cases, that might be pardoned, given that it was the perspective of a twisted psychopath). This story is effectively a script for an CW procedural drama. It looks fine on screen, but on paper, its charm is faded. I am talking Lee Child level , or maybe even Neil Gaiman; when they describe a situation or an action that should send the pulse racing, it does exactly that. With Mr. Mercedes, there is an overweight, suicidal cop, with two sidekicks - one too old to act like a teenager, and a teenager too young to act old. There's absolutely no way this unlikely trio - two of them with serious mental issues - would slip on a psychotic killer; it would probably have been a startler of a story if they hadn't.
The characters too, seem undercooked. With only one relatively straight-forward character in Jerome, I had a feeling King was far too concerned with sketching the darkness of Brady. So much so, that most other characters - Hodges included - got only passing character references. The book is teeming with characters, and never were they any different from what you would expect in a well, procedural cop show.
The best part of the book was the ending paragraph. While I can't call it a twist or anything, but the tantalising closure kind of makes up for all the lost opportunities. At the end of the day, as King put it himself, this is his first hard-boiled detective story. While I would definitely not call it at par with most other detective/crime thriller stories that I have read, if this does develop into a trilogy (the sequel Finders Keepers is already out there and the third installment End of Watch is due soon), there might be some way to bring something more to the characters.