WhatsApp beta 2.17.162 (and now 163) solves the issue of getting lost in a crowded conversation list by adding one very handy little feature: pinned chats, Android Police reports.
Tap and hold on any chat and you get a new pin icon in the action bar next to the delete, mute, and archive functions. Once pinned, a conversation stays on top of the list regardless of the recency of the last message compared to other chats.
You can only pin 3 chats after which you get a small toast notification message saying you can't pin more. If you change your mind, you can always unpin one of your 3 chats and choose another.
To get this option, you will need WhatsApp beta 2.17.162 or 163, which you can find on APK Mirror or grab from the Play Store by joining the official beta then waiting for the app to update.