Still have a first-generation iPod Touch running iOS 3. Works like a charm, can even download some apps from the App Store. Bit of a shock how both primitive and advanced the early versions of iOS were.
I had an iPad 1 running iOS 5 I think, but in the end I stopped using it because Safari would "crash" on most websites due to it running out of ram I guess.
IIRC there's 128M of ram on the fist iPad.