I asked for Shiver for Christmas a few years ago because it was raved about quite a bit. I thought it was a pretty good read. I read Linger months later, and liked it quite a bit more than Shiver (the world of Sam and Grace had already been built so there was more room for action). 
 I read Forever and liked it five times more than Linger because it was AMAZING. 
 I guess now would be the time for me to admit that for the first 25 pages of Forever I was utterly, totally confused. Forever is from four points of view, and so much happened at the end of Linger that I couldn't keep it all straight in my head. I ended up having to read a spoilery review of Linger to get the gist of it all flowing the right way in my head again (so I strongly suggest you reread or do a little research before starting this one). 
 Maggie Stiefvater's writing is simply beautiful. Even those that didn't care much for Shiver commented on how wonderful her writing is, and she does not disappoint in the final installment of The Wolves of Mercy Falls. It jumps us right to where Linger left off, and from there we are right back with Grace, Isabel, Sam, and Cole. 
 The middle of Forever got a little long for me, but that's my only complaint. The last third of the book flew by and had my heart pounding. Steifvater doesn't use predictable outcomes to satisfy the reader, which was my favorite part - she wrote this book in a real, true way, and it resonated with me as a reader. 
 It's not often that I love the last book in a series much much more than the others, but there is no denying that Forever surpasses both Shiver and Linger in my mind.