See the complete review in 102 Top Picks for Homeschool Curriculum.
I've seen many vocabulary resources over the years, but my favorite for general use remains the original Wordly Wise series, Books 1 through 9. (Books A through C in the original series are also available, but I think other academic priorities preclude their use in the early grades.)
I suspect the popularity of Wordly Wise stems from its effectiveness and reasonable cost as well as the fact that students can work independently most of the time. Books 1 through 9 are intended for grades four through twelve. However, vocabulary is somewhat advanced, so choose lower level books if your children are average in their vocabulary skills. I suggest starting average to bright students at fourth grade level with Book 1.
In this series, students use one list of words through four or five different types of exercises to become familiar with the word's usage in different contexts as well as its various meanings. Exercises include definitions, recognition of proper usage, word origins, prefixes and suffixes, analogies, and synonym substitution. Crossword puzzles at the end of each unit reinforce learning from earlier lessons. Children must truly understand meanings to complete the activities
The answer key for each level is relatively inexpensive and you will certainly want it to save time and energy since the exercises are often quite challenging....
EPS Literacy and Intervention has continued to sell the original series, but they will eventually take it out of print. They have already replaced it with the Wordly Wise 3000® series, and that series is already in its third editions. Wordly Wise 3000 retains many of the features of the original series....A variety of activities encourages students to think about vocabulary words and their meanings in ways that are more likely to help them recall them. A major difference is that the new series has added reading passages and some questions that require complete-sentence, original responses....
Book 1 requires direct teaching and differs significantly from the rest of the series. As with the original series Books A through C, I would probably skip it. However, I like Books 2 and 3 of Wordly Wise 3000 and consider them more useful than the original Books B and C.
The Wordly Wise 3000 series numbers books to correlate with grade levels. Book 1 is for first grade with the series concluding with Book 12 for twelfth grade.
The inexpensive answer keys for home educators for any of the Wordly Wise editions are not listed on the publisher's website, so you would do best to order through any of the homeschool distributors who carry Wordly Wise such as www.ChristianBook.com, www.RainbowResource.com, and www.ExodusBooks.com. You do not need the expensive teacher's resource books that are listed at the publisher's website. Test booklets and keys are also available for many of the books, but I don't think these are needed.