first: there is no such thing as a secure ftp server
now that we have that out of the way, i agree with n0c, that i only really use the one supplied wiht openbsd. another supposedly uber secure one is called publicfile and can be found here:
http://cr.yp.to/publicfile.html. I've also breifly used proftpd and liked it. one thing is for sure though, don't use wu! it has the most well-known history for security holes.
whatever one you choose, be sure to keep it updated to the most recent version, and i suggest creating ftp-only users. if the same users want to ssh into the machine, let them have another login and copy files from the publis ftp dir into their own dir if necessary.
also, do a search in this OS forum about using scp and sftp, there was a recent discussion on large file transfers that might be interesting to you.