I wish I could pick two on the "favorite" options. I'm partial to both the v2 and v3 for different reasons. I love the different characters on the v2 but the games are "meh" (except for slots) plus it's kinda "slow" to get to an adult due to the limited items/difficulty to obtain money. The v3 is nice for it's games and variety of items as well as being easy to acquire money but I often get redundant characters. (Getting the same character many times on certain gens.)
I would say my least favorite is a toss up of the v5 and v6. Though I liked the "freshness" of the v6, it was quite demanding and you were required to spend money on food to keep it sated. (Sure, this isn't hard once you get to gen 2, but many people who had non-US versions usually lost money with the parents leaving the baby in your care.) For someone that doesn't have time to constantly attend to a "superstar", this version had a steep learning curve for me. The v5 just didn't appeal to me as you took care of a family, not a single tama. It just wasn't my kind of thing and the characters were limited.