what is the best software based solution to secure the private key in Bitcoin
Hm. Rather strange question you ask.

First of all: it depends mostly on the infrastructure you use for your bitcoin-client.

If your OperatingSystem is compromised, you probably can't guarantee security of your privateKey at all. 

The Bitcoinclient supports a very strong encryption of the wallet (and ofc the private Key) since like 2 years natively. 

I guess if you are paranoid and want extra security, use a VirtualMachine, install some hardened Linux-OS (DebianStable?), there make a TrueCrypt-Container with a hard password and install your Bitcoinclient there.

Ofcourse, as mentioned earlier, you can't be more secure than making an offline-wallet. Remebering your key instead of writing it down could also serve as the absolute security feature. - but i guess you don't want to go that far? :)

i saved my keys in a TrueCryp-container encrypted with sha512 and used a strong password - after this, backup copies of this container wherever you want, so you don't lose it. sha512 is not even crackable for the NSA.