I'd say Meidotchi/Maidtchi. She was my first adult ever on my v4 and her close up animation was really cute!
Two of my favourite characters that aren't heard of very often are JALtchi and Tobiishitchi. JALtchi is a little aeroplane character that you can see on a v5 by using the aeroplane ticket, and he's also obtainable as a student on the Tamagotchi School.
Tobiishitchi is one of the customers on Tamagotchi Connexion: Corner Shop. He's a small, grey fluffy character and often you don't hear his name spoken.
I only recognise some, but I've probably had most of them, just cause I'm not good with Tama names. I know I've had Ponytchi, Pyonkotchi, and Tarakotchi before.
Anyways, I've always been a huge fan of Tarakotchi (I actually have one right now =P) so I voted him. I have a plush Tarakotchi and I named him Potato Greg X3 *hugs tarakotchi plush*