I've had a really good run of reading so far this year!
Since I switch back and forth between fiction and non-fiction I thought I'd double answer this question.
Street Gang by Michael Davis is probably my favorite non-fiction book so far this year. I don't know how much of this has to do with my past work experience in TV, but I thought this book was really great in the amount of detail the author was able to cover without the book ever becoming dry.
If you twist my arm, I would have to say Alex & Me by Irene M. Pepperberg was my least favorite non-fiction book so far. Although I have no regrets about reading it. The only thing that puts it at the bottom of the list is the writing. I feel like I could have written the book myself if I had had those same experiences. Still, it is a great story and well worth the little time it takes to read!
Moving on to fiction...by George by Wesley Stace is such a good book! Multi-generational, focus is on the young male character and a ventriloquist's dummy, but the women in the book are very well developed and strong.
And, least favorite fiction would have to go to The James Boys by Richard Liebmann-Smith. It's an ambitious book imagining Jesse and Frank James as brothers to Henry and William James. I really like the premise but the author put things together in a very confusing manner by including biographical research in a way that pulls the reader out of the story. Still, I will pick up his next book, if there is one.
I've linked the titles to my reviews if you're interested.