DVDStudioPro is more or less the only choice for professional DVD authoring on the Mac. For encoding MPEG2 files there are various solutions. We have very good experiences with BitVice, they support the Media 100 colorspace ("StudioRGB") and Media 100 i compression marks.
I agree, Apple's DVD Studio Pro is the best. But, if your on a tight budget or only do very basic authoring, Apple's iLife has iDVD (It even uses the same MPEG encoding as DVDSP) It's pretty darn amazing for practically no money.