That depends on what you want to do with it. If you want to be able to browse the web with it, I'd suggest going down either the Android or iPhone route. iPhones will be more expensive.
There are newer Blackberries that have touchscreens, but my understanding is that their app market is more limited. Of course, it also depends on what the carrier is offering as good deals at the moment.
The best advice is probably to go and play with the phone before buying. Go into a phone shop and ask to play with the phone(s) you're interested in, then tell them you'll think about it then go and buy online. ;-)