I'm a happy Android phone user, but not yet succumbed to atablet of any description. I confress that I suspect that sooner or later I'll end up with an iPad because, although I'm generally not fond of Apple products, the range of apps is hellishly impressive.
On my phone the single most useful app is "Aviation Tools Free", which is easily found and gives me notams, AIP charts and Wx per airport.
"Scientific Calculator" is a good facsimile of an fx83 and I use routinely for any arithmetic I have to do down route.
"Swiss Army Knife" is handy for control displacement checks (I do a few permit revalidations annually as an inspector" and for the torch.
After that, I mostly use it as a web browser - so have the usual met sites, etc. bookmarked.
W&CG I'm sure there are apps about, but to be honest I use a pen and pencil for that, or my GPS has a good page for that if I was too lazy to work it out longhand.
I know that GASCo are currently trialling a version of their take-off and landing performance calculator for the iPhone/iPad, but when I asked they're not currently planning to do anything for the Android.
G