Great question and as with you it's really hard for me to decide on just one. So.. here's some of my fave's that come to mind.
I've always enjoyed the winged shells of Angelgotch, they are just cute. So for shell design in particular of shaping, I'd chose that one.
The clear/see-through style has always been a favorite (blues in particular) on any type.
The yellow with black tiger stripes was my first pick and still I miss those stripes when they wore off Mimitchi Tama. I do have a Japanese one with a dragon on it, that too is a favorite =)
When talking solid colors the teal with blue border and the tan with black border come to mind right away. In drawings the blue with silver Mimitchi (p2) on it is quite nice too. For mini tama's I love the black with flames.
For an ocean... I do like that see-through one, but the teal one is real nice and it has the lil froggy guy on it =)
Mesu/osu I'd chose the clear and clear smoky pair for sure.
With connection... I never much enjoyed the designs on them, they are alright. The connection with hearts on it would be my choice there. I'm not a fan of pink, but I like hearts.
So there's a incomplete list of my favorites off the top of my head