As far as I'm concerned, iPhone and Android are both great options for most people. I would stay away from Blackberrys, Windows Mobile and Symbian phones, if only because the developer communities are so much smaller, leading to many fewer available applications.

Apple is pricey, but if you buy AppleCare and don't jailbreak, they're very, very good at making sure you stay a happy customer. They also tend to have better battery life than many Androids.