Montgomery County Public Libraries system is the best public library system I've ever been to. The one that I frequent the most is the one in Gaithersburg. The library itself is not that big, but it maintains a good selection of book collections. Even if you can't find the book that you are looking for, you can just go on to MCPL's website and do a search county-wide, and you can request a hold and they'll notify you once your book is ready. I use this option a lot and it has been serving me really really well. The library is also constantly acquiring new books; I can tell because I am always seeing new books during searches - just when I thought I've read all the books in a certain subject, new books are coming in! They also have a foreign books section that have a decent selection of books in foreign languages. The library also has a video section, now, which they added just months ago. The movie selections is not bad, and they also have limited selection on DVDs. From what I've observed, the selection for movies are better during the day, before they are all picked off. The staff at the library are on duty to help you if needed.
The only bad thing about this library is that it's often crowded, and people don't always keep the noise down.