I have 32 and it's not enough. I have to decide which apps I want on it and which music.
It depends on how you use it, but I have 20Gb of apps and 70Gb of music. You can't increase storage so buy bigger rather than smaller but until you get one it is very difficult to know. Games can be 1Gb if they have good graphics, CoPilot Europe is 2Gb, most apps are a lot smaller. Videos are also large.
Unfortunatly, you won't know what you want until you start using it so some people find 32 plenty, others find 64 to small, 16 is usualy regretted. YMMV