[kpmedia]

For the best quality, I'd do this:

1. Open FLV in VirtualDub, using the FLV Input Driver. Filter as needed, and a lot of FLV's could use some deblocking, NR, etc
2. Encode out to HuffYUV AVI, or frameserve to an MPEG-2 encoder (TMPGEnc Plus 2.5 works fine)
3. If HuffYUV AVI, encode to an encoder. Again, TMPGEnc is a good low-cost option. I use MainConcept.
4. Author to DVD, using your favorite authoring software. For example, TMPGEnc DVD Authoring Works.
5. Burn the disc in ImgBurn.

As far as an all-in-one tool, you're going to honestly get really crappy results. Mediocre at best. This easy 5-step process is mostly freeware, and you can improve the quality of the FLV along the way, to make the DVD look better.

You did ask for "best".