As has been said: depends on what you want and what you like.
-Like fooling around with trackers? Schism Tracker (free) is an Impulse Tracker clone. IT was famous in the nineties. Want to do a lot more than you can accomplish with Schism? Then there's Renoise (paid).
-Or, if you come from Windows and are already familiar with Fruity Loops, then LMMS (free) is an obvious choice.
-On the other hand, I personally dislike editing in LMMS and very much prefer MusE (free) for midi sequencing. MusE reminds me of old-school Cubase a lot, and is my preferred music creation tool.
-Then there's Ardour (free?), which is more suitable if you record live instruments a lot, and don't do much with midi.
-Or if you compose in notation, there's Rosegarden (free?).
-Specifically for sequencing drums, we have Hydrogen (free) as an option.
And then there are the (free and paid) plugins: synths, effects, studio tools, you name it. Take a look around on kx.studio for some cool free plugins. And check out U-he for both free and paid synths, ZynaddSubFX (free) synth, Pianoteq (paid) piano-and-more emulator.
And let's not forget a wave editor: I still use the wonderful vintage Cool Edit (with wine), but if you are so unlucky to not have a copy, then there's the linux native Audacity.