I didn't find any iPad app that's supporting samba.
What is the best way to access Windows shares from an iPad?
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Because there is no normally-accessible local filesystem on an iOS device, an app that "supports SMB" doesn't tell us enough. Because you can't simply copy files from a SMB share onto the iOS device.
Air Sharing is probably the most compatible for whatever you want to do, as it opens almost anything. Goodreader is primarily a document reader (txt, pdf, etc.), and OPlayer is a multi-format video player. It plays almost anything, MPEGs, AVIs, MKVs, etc.
What you want to accomplish beyond merely 'connecting to a SMB Share' will dictate the app you'll be able to use.
It's easy to hook up dropbox--not quite the same as you are asking but supported on pc/mac/iphone/ipad/android/linux/... and really does a great job of syncing your files together.
I haven't seen any other apps that would allow you to accomplish the goal you are after, but you will find some that will let you stream media from a certain type of media share--but that's not what you asked.
The files app on iOS and iPadOS can connect to network file servers directly so there’s no need for a third party app unless you want features they provide over Apple’s native SMB client solution.
(This wasn’t implemented when the question was asked, but it’s a great solution for now.)