Punch-Out!!, hands down.
Punchout's dual monitor setup kind of intimidated me as a kid.
That's funny, because the dual monitors are mainly what drew me to the game as a kid. 1984, I was 9, and the machine was brand new. It was just an awesome, towering sight. I had to look up to see the top monitor. Plus the white grid on the black control panel, the green wireframe boxer, the speech, the overall huge size of the graphics/sprites, and the way they could zoom in and out, were like nothing I'd seen before. The game looked ahead of its time to me, even at 9 years old.
Then the gameplay quickly hooked me. Nintendo was quite clever with the way they designed the difficulty of the game. The first two opponents could be beaten by most beginners using no strategy at all, just button mashing (I beat them that way the first time I played). Then there was the infamous Bald Bull, cursed by Punch-Out beginners the world over. Bald Bull could
almost be beaten with just button mashing, but not quite, which made it seem like if you had just one more try, you would beat him, though few people ever actually beat him by mashing buttons. Overall it was an effective way to hook the players, evidenced by Punch-Out being the most successful game of the year, and being known for having earning power that lasted longer than most other arcade games of the time.
I think this is the main reason that Super Punch-Out wasn't nearly as successful, despite being a better game (IMO). It was designed for people like me, who had mastered Punch-Out and were ready for more of a challenge. It was
not beginner-friendly at all; most people were trounced convincingly by the first guy. I was already an expert at Punch-Out when I first played Super Punch-Out in the '80s, and I was beaten by the first guy like everyone else the first time I played, though I progressed quickly after that because I already knew the basics from Punch-Out.