It's really hard to choose, since I love all of my odd pets for different reasons! The odder they are, the more charm they have to me. Back in the 90's when they started cropping up at every big box, convenient, and grocery store, I would get excited to see a new pet I've never heard of or seen before. It was also such an exciting feeling to open up a new pet and pull that tab out, because you almost always got something different.
I think if I had to choose my ultimate favorite, it would be my "chick-a-pet", which is the exact same as the chicknoch and bird watching pets (gyaoppi logic). I like that it takes on a a few different forms before its final stage, it requires just the right amount of attention, the animations a simple but charming, I like all of the adult forms, and the weather features plus the extra card style game was a bonus too!
I also like the Nakamichi pet for it's simplicty and ease to take care of. It is hard to neglect, and a lot different than any of the popular pets. The Electronic Pocket Chicken was also one of the ones I played with a lot because it had some cool features and fun games, and the adult forms were similar to original Tamagotchi characters. Plus I liked the big screen with big chunky chraphics, also there wasnt much animation to them.