Off the top of my head I don't know of any that have the option to be battery powered that sound that great, TBH. My wife and I picked up a Memorex dock from Walmart a couple years ago for about $30. It's not the best sounding, but it doesn't sound like garbage either. On the other hand, my mother has an iHome docking station that sounds amazing. However, I think she spent over $100 at Best Buy for it.
I'd suggest going out to some local stores that have them on display and try each one with your iPhone. As far as brands go, Altec Lansing , Klipsch, and Logitech are the top ones I'd recommend. Some Sony's are good as well.
Also, you'll want to make sure that it says it's iPhone compatible. I know initially there was an issue where the iPod docks wouldn't recognize the iPhone (because Apple switched some pins around in the connection). Most of them should now be compatible with both, but it never hurts to double check.