09 Cubase Steinberg's Cubase is now probably the main PC users' package.
It's got great features for controlling external devices like synths, samplers and effects boxes but it's not as intuitive as some of the other sequencers so you'll need to put in some hours to master it.
Some of the menu options aren't that easy to find and like all software, you'll get along much quicker once you learn a few of the main keyboard shortcuts.
Cubase is one of the original MIDI sequencing packages and its audio capabilities were added much later, when computers had evolved to be powerful enough to handle it.
Some critics of Cubase point out that it's still not as dedicated to audio as it could be but it's still a very cool piece of software.
There are various versions of Cubase, with different features such as the ability to produce musical scores from your MIDI sequences.
The entry-level version will set you back around £270 while the full version retails at around £450.