I am a fan of BIPs that have resulted from significant research and engineering effort and make the network better. That applies to a lot of BIPs, but especially BIP 327 comes to mind. But there are many others that meet that bar. A number of Pieter or Ava’s BIPs. The Compact Block Filter BIPs, Silent Payments, and Ancestor Package Relay. There is actually a pretty solid number of good BIPs—authors going the extra mile to ensure that their write-up is high quality and covers all the bases.