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I'd go with Kirby's Adventure. Adventure just left the biggest imprint on me. Kirby games are still trying to emulate it. It wasn't just good "for an NES game." It's still a great game now.

For coop, I like Return to Dreamland better than Superstar. There's not as much variety, but the powers are way cooler, and if sidescrolling Kirby is what you want, then there's a lot of content. I never played Ultra.

I wish Kirby's Adventure had been released in like the 80s rather than like '93 when the SNES was already out. Who knows what would have happened if people back then knew that the NES was capable of that.

I'm not saying there aren't other fun NES games at all, but Kirby just seemed a lot more modern than other NES games. It's totally easy to understand and play today and doesn't really seem all that outdated or retro (I don't know, maybe I'm the only one who thinks this), and it probably has the nicest graphics on the console. If it had a 2-player mode it would probably be my absolute favorite NES game.

Kirby games generally don't HAVE big difficulty spikes or even much difficulty. That's the brilliance of them, because games like Mario now have Super Guides for people that can't play them, while Kirby has always been easy enough for anyone to enjoy while not embarrassing them by showing them how to play it if they can't.