Out of the connections V3 and v4/4.5 are the best (imo). They're not difficult nor too easy to care for, the games are fun, even if the V3 ones go on for a while. V4 is where they introduced going to school and work and I really like that about V4. If you're going to get any connection version I'd say either of those.
V1 and V2 are kinda difficult in comparison, games wise.
I found V5 to be annoying, stats seem to go down really quickly and the whole bonding thing is irritating, plus mine went buggy sometimes and I remember a lot of people saying their's did too.
I don't have much of an opinion on the V6 as when I did play it I really didn't like it so I didn't play it for too long. The only redeeming factor of it for me was the sound block game.