Any Arduino or clone will do. Some will be better suited to your project, for some you might end pay premium for features you don't use.
If it's just tinkering around, make LED blink, drive a small display, attach a few buttons or some RC-servos, stick with the Duemilanove, the Uno or any of models with the same form factor so that the shields will fit. Those are also the most universally available and often come in useful starter kits. Also consider what is easily available, specially if you're located outside of the USA.
If you need small and aren't afraid of soldering a bit, consider the Arduino Pro Mini or the Lilypads, but be sure you get the ones with the right Voltage for you (5V or 3.3V) and don't forget the USB-to-TTL cable or board so you can attach it to your computer.
If you need more serial ports, more IO, more this or more that, the Arduino Mega might be the product of choice. Here, the default shields won't fit physically (but you can connect them).
If you're into some activity where already customised Arduinos like the ArduPilot, Rainbowduino, Seeduino Stalker or the Arduino Bluetooth exist, those options might be a sensible choice.
So you see, no good answer for you, you need to figure it out on your own. Best check what your preferred retailer stocks and what takes your fancy.