I'm not bang up to date with software, but I'd say Reason.
You can start very simply in Reason (just like Fruity Loops, with a step sequencer), but once you get into it and get some experience there is soooo much more you can do.
Once you get your head round the basics and want to take a step up 'pro-style' production techniques (playing more with effects, tweaking the synths etc) you can come up with some great sounding stuff. You can automate just about everything, so although some of the default sounds are a bit cheap-sounding, by the time you've recorded/drawn in a load of tweaks, then chained some effects (and you can automate all of those so easily), you end up with something altogether different. Just about any fader/knob you see can be automated, plus you don't just see them move, the ones you have automated are highlighted to show you. Viewing your controller tracks is so easy. Everything is so logical, intuitive and straightforward.
Plus if you then want to take it still further and get more expensive stuff on the go like Cubase and all the VST plugins you can use ReWire to output your Reason tracks directly into Cubase; then use really top-notch FX plugins and processors. Then Reason basically becomes your sequencer/starting point and you can then do all the finishing in Cubase, or export audio and finish that in Cubase.
It starts simple and is great for beginners but it is a deep, deep program that you can keep using once you progress - and it is so stable, has never once crashed on me. Plus, I love the way you see your virtual rack so you can 'see' what you're putting together. I started with Atari and Akai 1000 so I like that though
So yeah I think it's the tits!