what's the best app to play blue ray disks on an iMac?

VLC Player...

Since macOS does not natively support Blu-Ray playback, the best solution is VLC, an open-source video player that has been around for nearly 25 years. However, you cannot simply insert a disc and expect it to play. Video playback relies on libaacs, for Advanced Access Content System, the digital rights management system used by Blu-Ray and the now-defunct HD-DVD format.

https://blog.greggant.com/posts/2024/02/19/how-to-play-blu-rays-on-mac-vlc.html#:~:text=Since%20macOS%20does%20not%20natively,and%20expect%20it%20to%20play.