I use Panorama Factory, have done for years.
www.panoramafactory.com
Why? I suppose it's primarily because I've been using it since it was shareware with only a nag screen once the 30 trial had expired (v2). It served my purposes well for many years. I now use v4. Nice easy interface, intuitive workflow, quick and easy, not tooo fussy about alignment and overlaps, can't really fault it. You can do some great virtual reality tours, making full circular panoramic wraps in QuickTime format too.
Duncan