Samsung messages still has some RCS functionality, but they are abandoning their app. Google Messages is going to be the RCS standard that the carriers are going with. Best means different things to different people. You can somewhat customize Google Messages, especially with those contacts who also use Google Messages. You can't put distracting background art on it, but you can select from a limited color palette the colors for conversations. In the future, as RCS develops, there may be other apps able to integrate it that are more customizable, but I believe that Google owns the RCS technology and may or may not allow other apps to use it.
For future reference as to Google Messages and iPhone cross-platform compatibility with RCS, it will only work while using Google Messages. Any other messaging app, you will not have the ability to have full res photos and videos, send and read receipts, typing indicators, reactions. If you really need to customize your texting app to look pretty, then by all means, stick with the inferior product rather than the feature rich product.
Later this month, all of your friends with iPhones will finally get RCS support, and texting with them will be much improved.
I come from a Motorola phone and was using Google messages on that phone and with Samsung I didn't care for the app so I downloaded and am using Google messages again. In the past I also used Verizon messages+ app and that was a good app for me also.
