If you want the absolute best, it's Sonic Scenarist. It's very complex and costs something like $15,000-40,000 to actually purchase, but it's what the big studios use to produce commercial DVDs. Obviously, it can be found through other channels like most software though. Without using guides at doom9, etc. I never would've gotten anywhere with it, so you may want something a little more suited to a home user that automates some of the process.
For that, I'd recommend looking at
Mediachance DVD-lab or maybe one of Ulead's packages (DVD Workshop or MovieFactory). Never used Ulead's authoring stuff much but for video editing I'm a big fan of their MediaStudio. The normal difference between basic and pro packages is stuff like multiple 5.1 audio tracks, subtitles, angles, complex animated menus, etc.
I don't know of any totally free software (except IfoEdit, but that certainly doesn't count as having a lot of bells and whistles), except maybe if you already happen to own Nero 7, you could try Nero Vision.