I'd nearly written an essay on this, but this may be better:
Apple iPhone 4s - If only for a few apps I want to use (that aren't available on Android). Already dated though, so would I want it for two years?
HTC One or One mini - If you read the reviews it's supposedly the best phone out there! Really?
Sony Xperia Z - Looks good on paper but I've heard about problems about the screen cracking (I guess that could be said about the iPhone too).
Samsung Galaxy 4 in some form or another - Not sure why this isn't higher up the list.
I can get any of the above phones for a similar monthly fee, with unlimited mins, at the same price as my current phone...my phone is supplied to me, so I can't go over that amount.
Cheers for any advice.