In my opinion these are the main features of the four Vista versions;
"Does what it says on the tin". You get DirectX 10, but no Areo or any of the other main features. This version isn't worth it at all, despite being the cheapest, in my opinion.
A good all round choice. You get all the main Vista goodies and Media Centre software built in. There's even an MPEG2 codec packaged with the OS so you can watch DVD's out of the box. More expensive than Home Basic but worth it.
Another good choice for those who aren't bothered about Media Centre software, but want the built in file encryption and Remote Terminal Services Client. Neither of which comes with Home Premium. I chose this version myself as I use PowerDVD for the MPEG2 code and most remote control bays come with Media Centre Software (Creative Live Bay). Usually similar in price to Home Premium.
This version includes everything from Home Premium and Business along with BitLocker and Dreamscene, though I don't think that those two tools are worth the large price premium.
In a nutshell, I think the best choices are either Home Premium or Business, depending on weather you're happy to use third party Media Centre software and MPEG2 codec or not.