Windoze is the default choice for those of us that are so accustomed to it it
FEELS the easiest - which is not to say that it actually is ..and let's not even get into how safe it is (think security breaches, etc. here) to leave unattended in the hands of someone who has no idea of how to defend it against the depredations of those that would fUxx0r her PC (and leave you to fix it) ('scusi, I'm on a bit of a Big Word buzz)

SO: granny-friendly is not only easy to get to grips with, but also safe and manageable ..and when you factor in $$$, ol' 'doze starts to look not so good.
My 2c worth: (not a fanboi, being pragmatic here) the latest Ubuntu - yes, the Gnome one, not that XP interface knock-off that is Kubuntu.
Think about it:
- it has the basics of a desktop to use
- the UI is hugely simplistic - IOW not daunting to said granny
- the UI behaves consistently, a huge plus
- viruses, spam/phishing that comes in by email that granny mindlessly clicks on? *pfft*
- the ability to add apps as she gets to grips with it
- cheaper ..and notably on the hardware too
- bragging rights: having a granny that does Linux!