It depends on how much control you want in your slideshow. iphoto and iDVD are both easy and give you transition options, fit music options and some basic controls. I like iphoto though because for me iDVD is a little sluggish.
iMovie is good if you need to edit the length of individual pictures or more than one type of transition or adding titles.
Another app you might be interested in checking out is Boinx Fotomagico. It's shareware (US$79) and though it gets only 3 stars from versiontracker, I've seen some well done presentations with this, and it plays well with the iLife bundle. http://www.versiontracker.com/dyn/moreinfo/macosx/23778