Unabridged Opinions: list one...a series...

so each list will have a different purpose...so there WILL be repeats, but hopefully you will be able to use the lists dynamically (ie if you need Sci-Fi recs..go to that list or if you need specific level recs..go to that list) this list will be.."series"
1) Of course, JK Rowling's Harry Potter series must be on the list. The beauty of the series (besides fantastic stories and writing) is that it actually increases in difficulty and length as it goes..so the first book is both the shortest and the simplest to read. If students are turned off by how long the last ones seems, the first one is, in reality, quite managable. Plus, good, well-written fantasy fun.
2)JRR Tolkein's Lord of the Rings series is another amazing one. These are actually fairly difficult (especially The Two Towers, the second in the series), but those students who have seen the movies may be able to access the books even if they are not high level readers--and it is always good to suggest The Hobbit which was actually written as a lower-level children's book.
3)Another fantastic series is Garth Nix's Keys to the Kingdom set. These books, fantasy about a boy who is given a key that allows him to essential travel across dimensions, combine really tight, effective writing with an exciting storyline. Each dimension is represented both by a day of the week and a level in an old, mouldering house. Highly suggested for students who are low to mid level and enjoy fantasy or adventure. (Garth Nix has written MANY series, but this is one of his best, and a good intro to his style)
4) Lemony Snicket's inventive A Series of Unfortunate Events is very popular with readers of all levels. Although the humor (and the stories) can get repetitive, the kids love them. A great "doorway" to reading series. Plus, the author writes back.
5) The Darren Shan Cirque du Freak series is an original and engrossing vampire series. Definitely accessible to mid-level, and some low-level, readers--these are good books for students who want a book high on actions without too much slower descriptive sections. Darren, also the protagonist, is easy to sympathize with and his adventures are exciting.
6)Anne McCaffery's Pern Series actually contains about five individual series under one heading. Each of them are good, however, the two earliest (the Dragondrums trilogy and the Dragonflight trilogy) are the best.
7) Jenny Nimmo's Charlie Bone series is of a little higher level than the Lemony Snicket books, but still a good doorway for more reluctant readers. Definitely written to appeal to the "Harry Potter set" of readers, Charlie is a young boy with magical talents who has adventures. But, a good series.
8)Brian Jacques Redwall series is just fantastic. The characters, all animals, have spectacular personalities and the battles, and the feasts, are wonderfully described. Highly suggested for mid-level readers, low-level readers should be able to lock in on the great stories as well. As an aside, has a great "fan club" and website.
9) The Sisterhood of the Traveling Pants series (now a trilogy, but with evidence of more to come) is great for the low-to-mid level reader. Especially appealing to girls--a funny series of books.
10)Protector of the Small, a series by Tamora Pierce, it's a fantastic series with a strong female protagonist--a girl searching to be a knight. Pierce has written a number of other series; they are good, and this one is one of the best.
11) Meg Cabot's Princess Diaries is a sweet lo-mid level series. Uncomplicated, but enjoyable.
12) Madeleine L'Engle's Wrinkle in Time series is one of the best of all time. The original trilogy is fantastic (and one of the first sci-fi's to have a strong female lead) and each succesive book is also wonderful. Her Austin Family series is also fantastic. Highly suggested for mid to high level readers. Low level readers may enjoy her first book, the stand alone And They Were Young.
13) Stephen R. Donaldson wrote two separate fantasy series. His first, the complicated (and heavily allegorical) Thomas Convenant series is really only appropriate for high level readers. His second, the A Man Rides Through series (actually only two books) is easier to read and understand for mid-level readers.
14) C.S. Lewis. All levels. Wonderful.
15)Orson Scott Card's Ender's series and the corresponding Shadow series are amazing. Ender's Game lives as one of the ultimate of sci-fi classics. Both Ender's Game and Ender's Shadow are appropriate for all reading levels. The rest of both series can become rough going for anyone below mid-level reading.
16) Isaac Asimov's Robot and Foundation series are not to be missed. Best suited for mid to high level readers--they are both classics in the sci-fi genre and just wonderful books. High, highly recommended.
In addition, it should never be forgotten that fantastic audio versions of many of these books exist. The Harry Potter series is incredible on audio, as are many other "classics." This is often an excellent way to introduce a child (or an entire class) to a book.