It's truly a tie between Coffretchi and Pianitchi but I also love Furifuritchi, Milktchi, Lovelitchi, Melodytchi, Moriritchi, Candy Pakupaku, Miraitchi, Clulutchi, Yumemitchi, Kiraritchi, Chamametchi, Hapihapitchi, Momotchi as well!
ginjirotchi will forever hold a special place in my heart as my first ever tamagotchi(rest in peace martin, i havent been able to get a p1 to 25yrs ever since you died)