My favorite graded reader so far is Sunnyvista City , an Oxford Storylines, level 3, by Peter Viney .  The plot and sentences flow smoother than any I've read.  Even with a vocabulary of only 1000 words, the actions and explanations are clear and succinct.  The dialog is simple but crisp.  There are enough characters to flesh out the story but not so many as to overwhelm us.  The mystery unfolds at just the right pace, not so slow as to lose our interest and not so fast as to lose our comprehension.

China Bookworm review

(part of a long comparative review of graded readers)