I think Pocket Frogs is in a category of its own. So many games these days rely heavily on in-app purchases, which is incredibly frustrating. Pocket Frogs offers in-app purchases as well, but they aren't necessarily required for gameplay like in so many other games.
I do enjoy Tiny Tower but it isn't quite the same game. Tiny Tower is more about completely random luck than purposeful and directed breeding. Don't get me wrong, Tiny Tower is a great game and does share some similarities, but the feel of the game is different.
And as for Pocket Butterflies...what a complete rip off! It couldn't be a more blatant copy of Pocket Frogs. I am a bit weary, though, of carbon-copy applications with descriptions written in poor English and nothing but 1-star reviews...
I've started Tiny Tower too on iMikos's recommendation, as well as a few RL friends. I thought I was so done with games in this format after a couple years on those Facebook games but this still hooked me. Like Pocket Frogs it doesn't punish you for logging in late etc which is great.