Suggesting a "smartphone" is too personal. Every user has his/her own preferences.
Some are pure communicators. They want to Text, Email, IM efficiently and quickly. Some like touch-screens, some like keypads. Iphone VS BlackBerry, vs Android.
My advice? Get some in your hands and try them. Decide what you like about them and don't like about them.
I've got a BlackBerry for Business because my main goal is to Email quickly and efficiently. I do some Text and some IM, but mostly email. BlackBerry suits that well. Plus, a few of my business contacts have BBerries and we BBM (BlackBerry Messenger) back and forth a bit.
I've got an Android as well for my personal phone. I like it. I've had an iPhone. Liked that too.
I'd not want to recommend anything. You have to decide what works for you. With all the changes in technology, whatever you get, you'll probably get something new and improved in a year anyway.
For pay as you go, I know Virgin has a pretty good deal. $35 buys you 300 anytime minutes, and unlimited Android use including TEXT and IM and Email. They are looking at throttling back in March for heavy users, but still, that's a good deal.