Content Coverage: The best GMAT preparation materials should cover all sections of the exam comprehensively. The GMAT consists of four main sections: Analytical Writing Assessment (AWA), Integrated Reasoning (IR), Quantitative Reasoning (QR), and Verbal Reasoning (VR). Ensure that the chosen resources address each of these sections adequately.

Credibility: Look for study materials developed by reputable publishers or organizations with a track record of producing high-quality educational content. This includes official GMAT guides published by the Graduate Management Admission Council (GMAC), the organization that administers the exam. Official guides are often considered the most reliable source of practice questions and strategies since they are developed by the same entity that creates the GMAT.

User Reviews and Recommendations: Read reviews and testimonials from other GMAT test-takers to gauge the effectiveness of different study materials. Websites such as Reddit.com, GMAT Club, and other forums dedicated to GMAT preparation are valuable resources for gathering insights from past test-takers. Pay attention to feedback regarding the clarity of explanations, relevance of practice questions, and overall helpfulness of the resources.

Adaptability to Learning Style: Different individuals have different learning styles, so it's essential to choose materials that align with your preferred mode of learning. Some people may benefit from structured study plans provided by comprehensive prep courses, while others may prefer self-paced study using books or online resources. 

Consider whether you learn best through reading, watching videos, practicing with interactive quizzes, or a combination of these methods.

Practice Tests: Practice tests are crucial for familiarizing yourself with the format and timing of the GMAT exam. Look for preparation materials that include multiple full-length practice tests, preferably in a format that simulates the computer-adaptive nature of the actual GMAT. These tests will help you assess your progress, identify areas of weakness, and build test-taking stamina.

Additional Resources and Features: Evaluate whether the study materials offer supplementary resources such as video lessons, mobile apps, flashcards, or access to online forums and support communities. These extras can enhance your learning experience and provide additional opportunities for practice and reinforcement.

Value for Money: Consider the cost of the study materials relative to the features and benefits they offer. While some resources may come with a higher price tag, they could provide comprehensive coverage and valuable support that justifies the investment. Conversely, there are also budget-friendly options available that offer quality content without breaking the bank. Compare the prices of different study materials and weigh them against their perceived value in helping you achieve your target GMAT score.

Based on these criteria, here are some of the best GMAT preparation books and online resources:

Official GMAT Guides: Published by GMAC, the official GMAT guides are considered essential resources for GMAT preparation. They contain real GMAT questions, comprehensive explanations, and valuable test-taking strategies. The Official Guide for GMAT Review is particularly popular among test-takers and is available in both print and digital formats.

Manhattan Prep GMAT Strategy Guides: Manhattan Prep offers a series of comprehensive strategy guides that cover each section of the GMAT in depth. These guides provide detailed content review, practice problems, and access to online resources, including additional practice tests and interactive study tools.

Kaplan GMAT Prep Books: Kaplan's GMAT prep books are known for their clear explanations, effective strategies, and ample practice opportunities. They offer a range of study materials, including comprehensive guides, workbooks, flashcards, and access to online resources such as practice tests and video tutorials.

Veritas Prep GMAT Books: Veritas Prep's GMAT books are highly regarded for their comprehensive coverage of GMAT content and effective test-taking strategies. They offer a structured approach to GMAT preparation and include access to online resources such as practice exams, interactive lessons, and a question bank.

GMAT Club: GMAT Club is an online platform that provides a wealth of resources for GMAT preparation, including forums, study plans, practice questions, and reviews of test-prep materials. It's a valuable community where test-takers can share insights, ask questions, and receive support from peers and experts.

Magoosh GMAT Prep: Magoosh offers a comprehensive GMAT prep course that includes video lessons, practice questions, study schedules, and full-length practice tests. Their adaptive learning platform tailors study plans to individual strengths and weaknesses, making it suitable for self-paced learners.

Target Test Prep: Target Test Prep specializes in GMAT quant preparation and offers a comprehensive online course focused specifically on the quantitative section of the exam. Their course features in-depth lessons, customized study plans, and a vast question bank with detailed explanations.

GMATPrepNow: GMATPrepNow provides video lessons covering all sections of the GMAT, along with practice questions and tips for test day. The videos are designed to be engaging and easy to follow, making them suitable for visual learners.