It all depends on your budget. If you have a lower budget; there's FL Studio and Reaper. If you have a higher budget; there's stuff like Reason, Ableton Live and Cubase. Personally I've only used FL Studio and it's great if you want a program with an easy to use piano roll. It goes for about $99. Combine it with a couple good sample libraries and you can pretty much create any type of music.
While Young is mostly correct, you need to factor in buying 3rd party libraries and even some 3rd party plugs when using Reaper. It does have some built in plugs but no built in sounds. This is where the cost savings (when comparing Reaper to other DAWs) actually gets skewed. FL Studio does have lots of it's own sounds but if you wanna buy a large amount of sounds then you could end up spending just as much if not more than other DAWs.
Well that's kind of what I meant when I said sample libraries. Personally, I don't even use the default sounds that much; but I use Sampletank 3 instead. You can get the SE version of that for $99. Reaper is okay too; but I prefer FL Studio's piano roll. FL Studio is easy to compose with and when you combine it with an external sample library VST; it's awesome.