If you're looking for something inexpensive yet complex and likely to hold your attention, I'd go for the V4.5. It's a great version, with very fun games, and your Tamagotchi can even get jobs and earn a salary. I'm not too fond of the characters on it, but a lot of people like them.
If you want a dependable, pet-like Tamagotchi, go for the V3. That was my first Tamagotchi, and it's the version I go back to when I want something easy but still engaging.
If you want a really cool experience, get the iD L or Tamagotchi P's, which have color screens, but are very expensive and predominantly in Japanese (There's an English iD L, but it's very rare and expensive).
But you really can't go wrong with most Tamagotchi versions. They all have a fair amount of greatness. Even the V5 (which is not my favorite due to its simplicity). There is honestly no Tamagotchi to hate!
Lots of the connections are expensive now.