Alright it depends on what you want to use it for and your budget. Most people would say, get the iPhone, the iPhone is the best, it's the best smartphone ever, and yeah...I would agree as well (I have an iPhone 3GS myself), however if your after something else, I would recommend something that runs on Google's Android software.
The iPhone led to the smartphone application boom however the Android market is really catching up. If I had to recommend a smartphone other than the iPhone it would be a HTC; either the Wildfire, Desire or Hero.
The Wildfire is great if your on a budget because it only costs $350 (Australian price) outright. Just like any other Android phone minus the price.
The Desire and Hero are great iPhone alternatives and although they are more expensive compared to the Wildfire, it is worth the money as they come with better specs and cameras.
Overall my suggestion would be:
Smartphone recommendation: Apple's iPhone (3GS or 4, nothing older)
Alternative recommendation: HTC smartphone that runs on Android (Wildfire, Desire, Hero)