If FreeBSD, then Atheros imo. If OpenBSD then it doesn't matter if you don't need WPA, otherwise I've made good experiences with Ralink. Well of course you should inform yourself about supported devices.
The best way to do it, read the manuals:
e.g. for Ralink/OpenBSD:
http://www.openbsd.org/cgi-bin/man.c...86&format=html
or
Atheros/FreeBSD:
http://www.freebsd.org/cgi/man.cgi?q...SE&format=html