What is the best for HTML5 apps? iOS or Android? I'm looking for a cross platform app.

XAMARIN is a good option for .Net Developers since, it has a tie-up with Microsoft. But, it's paid.

What application you want?

If you need content app, or just demo app, and platform design guidelines are not important, so you can take advantage from cross platform frameworks such as PhoneGap.

Be careful when using cross platform frameworks, be careful from their weakness, a lot of important native APIs are not available here.

If you need a good app, app for a customer, develop native.