T9 was incredible; it's one of my favorite pieces of technology. That said, I had a Palm Treo for a long time and that is my all-time favorite typing experience on a mobile device. The slightly rounded keys allowed great accuracy and tactile feedback (both for the "click" and for sensing where the correct keys were without looking).

I have now had an iPhone since the first generation and I still suck at typing on one (on the 3GS now). Auto-correct seems to hurt me more than help even after earnest efforts to trust it and stop hitting backspace.