See the list of .FLV converters to learn what software can be used for converting Flash FLV videos.

You may retain high quality with many codecs. Target size and format significantly affects the quality.
I prefer to have MPEG-4 Xvid .AVI videos (I use 2-pass encoding with 2Mbps bit-rate for DVD videos) or MPEG-2 DVD videos.

I could also suggest a MPEG-4 AVC codec (such as x264 or Nero Digital), but not for a standalone DVD player ('coz I haven't seen any DVD player supporting it).