Depends on user requirements.
I would recommend WavePad and MixPad. They're both very easy to use and you can try them for free from here:
http://www.nch.com.au/wavepad/index.html
http://www.nch.com.au/mixpad/index.html
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My personal favourite is Cubase on PC
others include, ableton live, logic pro, cubase, reason, fruityloops, adobe audition, etc etc
but... this is a question that, as you discover more about music production, you will realise more and more has no real answer!
eg, my choice is based on many factors,
- i only have a PC so cannot use eg. Logic, although i believe it to be excellent,
- I learnt how to use cubase from scratch at university
- i am very used to cubase now.
- it hosts vst plugins which can be found as freeware to a high standard
i have used ableton reason and fruityloops briefly for instance, but it did not suit me, but i know many amazing producers use it.
the nature of your question suggests to me that you are just starting your journey into music production, as such, you have two choices :
an easy to use program which does alot of work Behind the Scenes for you such as reason, garageband or fruityloops - great if you have ideas and just want to get started. while platforms such as this have been used to produce the most banging tunes of the moment, they offer me personally less versatility in my style of production
the other option is to go for something like cubase or logic, which are more of a blank canvas (although logic comes with many pre-made loops to play around with) some grounding in the use of midi, and audio editing is useful, but will make you a better producer.....
Maybe think of this question instead...
what is the best kind of producer? - one who needs the 'best' software, or one who MAKES the best out of what he has?
like i say, you are embarking on a journey, over many years you will still be learning, and new software will have appeared,
suck it and see