This depends on so many factors. What do you intend to do with it? How much money do you want to spend? Are you comfortable with soldering? Can you wait for a month for shipping from China?
However, most people who are just trying things out tend to opt for a cheap Arduino Uno clone, If you prefer a genuine Ardiuno, the Leonardo might be a better choice - it's a bit cheaper than the Uno and also a bit cleverer. For simple experimentation there is not a lot to choose between the basic Arduino boards, though - they all use the Arduiino development UI and the same "wiring" language for programming, and they are all pretty much compatible with the same extension boards ("shields").
Although you can use Arduino and Raspberry Pi for some of the same kinds of things, they are really very different.
Arduino is best treated as "programmable hardware" - it has a good selection of IO pins and (once programmed) will run its single program whenever switched on. Raspberry Pi, on the other hand is much more like a "proper" computer - it loads an operating system (typically Linux, but others are also availabls) from an SD card, and offers all the screen, keyboard, USB and networking you might expect.
Hi Frank - I have decided to buy a arduino leanardo. I don't get to work with circuits as a s/w dev. I have worked with breadboards in the past and can't wait to get started tinkering around with electronics again. Thanks for the pointers :-)