The most recent Japanese-language version is the Tamagotchi-iD. It's available in toy shops in Japan, or from sellers on e-bay.
The most recent English-language version is the Tama-Go. So far, it's only available from online shops such as amazon.com and toysrus.com. I think it will be available in regular toy shops in USA very soon, maybe within a few weeks. After that, you should be able to buy them from sellers on e-bay.
About "which is the best" - the answer depends on your personal taste. The Tamagochi-iD has a full-color screen. The recent Japanese versions don't have a Tamatown type of web site. However, the Tamagotchi-iD has many shops and places your Tamagotchi character can visit, built-in. Also you can transfer new foods, games, and wallpapers to the Tamagotchi-iD via certain models of cellphone with IR. The Tama-Go doesn't have this IR transfer capability, but it can access new content (foods, toys, games) while you've connected an expansion ROM (called a Character Figure.) The Tama-Go has a 4-shades-of-grey screen. There is a Tamatown web site for the Tama-Go.