Depends really. Look at your schedule first.
Busy? Go for the Nano. It's simple, cute and very portable.
Not really hectic schedule? Then go for the either the TamaGo or the Angel.
The TamaGo is simpler as what I've been told compared to the prior versions. It's better with figures though. So if you want your experience to be more fun, get the figurines as well so it wouldn't be too simple for you.
As for the Angel, it's simple as well. Like all vintages are. There are very cute characters. One of the best Growth Charts I have seen too. There are cute animations as well but if you have more free time, choose the TamaGo instead since the Angel has only one mini-game and no other options other than the basic ones.