Well FLStudio and Reason are better known as all in one Studio packages, where Cubase is more of a Pro Sequencer package.
FL Studio:
Pretty much all u need, loads of effects come with and some included synths and stuff.
Can record Audio
Full automation
Use of Midi
Full VST and DXI support for 3rd party products and Rewire support
Reason:
Pretty much the same as FL but u cannot record audio in the program itself
Theres no support for external VST or DXI so u get what it comes with
Also Reason as a rather difficult to undesrtand and learn approach at routing different Instruments and modules to create vast sounds.
Also supports Rewire (in fact i think they invented the Rewire technology)
Cubase:
Well a top end program if i may say so.
Pretty much does everything as Reason and FL Studio.
Been around longer than most other programs therefore its got a bigger user base.
Supports Rewire so any package that includes Rewire can be used within Cubase and vice versa.
Very good program for midi and external real Gear like Synths, etc...
My opinion, its not the software u use, but your creativity and knowledge that count. Ive tried all 3 of these programs and love all 3 but can work fastest and best on FL and Cubase.
I love FL mostly due to the ease of use and quickness it allows u to compose in. Just wish they made a few adjustments to the program like the audio recording, too much cliking here and there before u can actually record.
Then i would also love to see a better Plugin organiser more like Cubase where u can set ur plugins to show as Drums, Synths , etc....
Reason i think is a great looking software, sounds great but i personnaly hate working in it.
Cubase, oh yes Cubase you either hate it or love it and im stuck in between, lol,...
Its a great program but once u work with FL Studio u just wonder sometimes why Cubase cant for example have a simple Sample track like FL wich would allow for much quicker experimentation and in fact drum construction. Otherwise i actually use Both and FL Studio and im rather happy when i Rewire FL into Cubase SL3...