The newest American model is the Tama-Go (but I'm not sure I recommend it as the site where you can play games with the Tama-Go, enhancing it, is down, indefinitely or permanently). You can find it for super-cheap on Amazon.com. Black-and-white display, nothing special.
The newest Japanese model is the Tamagotchi P's, but they use add-on game cartridges to enhance gameplay, they are pretty expensive. Color display, I hear very cool.
A fairly new model is the English Tamagotchi iD L. It is very expensive. Color display, I hear they're cool, too.
The iD L in Japanese is slightly older (but not old) and is pretty expensive. Color display, I have one and it's awesome (except it's in Japanese which does not affect gameplay)
The Tamagotchi Nano is a fairly newish Japanese tama, it is a black-and-white display and is a fair price. Easy to care for, I hear.