Heirloom mailx on OpenBSD because native mailx has no MIME support. On my
http://sdf.lonestar.org/
which runs NetBSD on Alpha I use native mailx since NetBSD mailx does have a native MIME support. Merging those twelve files from NetBSD to OpenBSD which provides MIME support for mailx has been now for a while on my todo list. Additional benefit of Hairloom mailx is that can fetch mail from IMAP and POP mail boxes.
P.S. Yes, I am aware that I can add MIME functionality on native OpenBSD mailx with
/converters/mpack or mail/metamail. I also like /mail/fdm
http://fdm.sourceforge.net/ much better
than fetchmail or getmail for fetching IMAP and POP mail boxes.