WAToolkit is the best rated extension for WhatsApp Web on the Chrome Web Store. Please see the full description on the Chrome Web Store itself: https://chrome.google.com/webstore/detail/watoolkit/fedimamkpgiemhacbdhkkaihgofncola
It has a rating of 4.7 stars out of 5 (3,800 votes), and 66,000 users. All of the users gained with zero expense on advertising, just by word of mouth organically through the Internet.
The functionality itself is very simple, and thus easy to maintain and to expand by any developer with some knowledge of Javascript and HTML (I could maintain it for a price as well).