Best: Super Mario World, because it actually had action on it. The screens most people are thinking of are either the demo movies that play after the title screen (Mario Kart Wii) or the file select screen (Mario Sunshine).
Worst: hmm.. For console games, it's a toss up between Mario Kart Wii (mabey the development team spent too much time visiting YTMND) and SMB2 (curtain background with characters on it that don't move! Exciting!). If I have to choose just one, then Mario Kart Wii, because I was too young to care what the SMB2 title screen looked like.