That's really tough... I love my V1, my first Tama ever. My V2 is great, but I haven't played it in a while, like my V1. My V3 and V4 are both really great, and I don't own V4.5, V5, or V5.5. But my Music Star is amazing (except for the button lag, but it's really fine). I'm ecstatic about the Tama-Go, and I'm confident I'll love it, so maybe that one... I'm having a Japanese Angelgotchi shipped to me right now, and I prefer the Japanese one to the English one for some reason. Ehh, I might choose that one anyway, practically the same, and I love Tamagotchi Angels! I've recently been interested in an Ocean, English preferably because of the designs, so that could be my favorite, too, since I know practically everything about it. This is difficult!
So my favorites are the Music Star (you just can't get over how great it is!), Tama-Go, Angel, and Ocean. For the poll, I voted for Ocean (and was the only one
) because I just love it so much... It's definitely a tie with the others, though.