What is the best diamond version needed to be used as requirement ?
There was an internal change in DIAMOND in Feb 2020 (Build 134) that causes an incompatibility between DAA files produced with DIAMOND with build <134 and later versions of DIAMOND, effecting only alignments produced in long-read mode.
The very latest release of MEGAN, 6.19.1, can process DAA files produced by all versions of DIAMOND (both old and new).
Early releases of MEGAN will fail to process DAA files produced by recent DIAMOND releases (build >=134). The program will throw an ArrayIndexOutOfBoundsException.
So, to answer your question: while it is usually best to always use the latest version, the latest version of MEGAN does not require a DIAMOND update.