There are many Online Coaching Institutes which are equally good when it comes to CAT (Common Aptitude Test) Preparation. Below, I shall list some online institutes along with there specialties, then it's your call which one to choose -

• For the best Quantitative Aptitude and Data Interpretation and Logical Reasoning - Elite Grid Academy
• For best concepts in Verbal Ability and Reading Comprehens ion - Wordpandit
• Mocks by Triumphant Institute of Management Education (TIME)

To get tips for CAT Preparation, you may visit the link - How to Prepare for CAT Exam 2021 - CAT Preparation Tips for 2021 Exam (careers360.com)